Working rotations at sea complicates the answer to a simple question: where you pay tax. This article explains the concepts that structure that answer — tax residency, treaties and documentation — without going into figures or specific cases, which are always a matter for an advisor.
Few conversations come up as often in a mess room on board as the one about taxes. Someone says they pay nothing, someone else says that's impossible, a third person cites the case of a crewmate from another country, and the discussion ends with no conclusion. It makes sense: rotational work turns a question that's simple ashore — where you pay tax — into a matter with several moving parts.
This article doesn't resolve specific cases, and it can't. It's general information aimed at helping you understand what concepts are in play and what questions to ask. The answer for your situation depends on your country of residence, your contract and where the service is provided, and that answer has to come from a tax advisor who can look at your documentation.
Tax residency is the starting point
If there's one idea to take away, it's this one. Tax residency is the criterion that determines which country has the right to tax your income, and to what extent. Everything else — treaties, withholdings, exemptions, deductions — falls into place from there.
It's important to understand that this is its own tax concept, distinct from other things it's often confused with. It's not nationality. It's not the country on your passport or where you hold a residence permit. It's not necessarily where you bank or where your vehicle is registered. Each country defines in its own law what requirements make a person a tax resident, and those requirements usually combine several elements.
What kind of criteria are used
Without going into specific thresholds, which vary from country to country and aren't worth generalising, the usual criteria revolve around three ideas. The first is physical presence: how much time you actually spend in the territory. The second is your centre of economic interests: where your main sources of income and your assets are. The third is your personal and family ties: where your family lives, where your usual home is, where your life is.
The combination of these criteria explains why two divers with similar rotation schedules can end up in different situations. One with family and home in their country of origin will struggle to stop being resident there just by spending time offshore. Another who has moved their whole life to a different country is in a different case.
Why counting days isn't enough
It's the most widespread mistake in the industry. The idea circulates that there's a magic number of days away that automatically settles the question. Physical presence is one of the criteria, but it's rarely the only one, and shedding tax residency in one country isn't just about being absent: it usually also means proving residency somewhere else and undoing the ties that keep you linked to the first one.
Double taxation and treaties
The second concept comes up when two countries, each under their own rules, both consider they have a say over the same income. One because you're resident there, the other because the work was carried out in its territory or for one of its companies. That's double taxation: the risk of the same income being taxed twice.
To avoid this there are double taxation treaties, bilateral agreements between countries that divide up the power to tax each type of income and set out correction mechanisms when both retain some right to it. These are individual treaties: what's agreed between two specific countries doesn't have to match what another treaty says, and in some cases there's no treaty at all.
What to ask about this
The useful questions to bring to an advisor are reasonably specific. Whether a treaty exists between your country of residence and the country where the service is performed. What that treaty says about employment income and, where relevant, about work aboard vessels or installations, which sometimes gets its own treatment. What correction mechanism applies. And what formal obligations apply to you even if you don't end up paying in one of the two countries, because filing and paying are different things.
Employed or invoicing: two different scenarios
The legal form of your relationship with whoever pays you completely changes the picture. It's a decision many divers make out of convenience, or because it's what the agency offers, without seeing its consequences.
Employed work
If you're an employee, there's an employer who takes on a set of obligations: withholding tax where applicable, making social security contributions where appropriate, issuing payslips. Your administrative burden is lighter and a good part of compliance is handled by someone else on your behalf, although ultimate responsibility for your tax return is still yours.
The point worth looking at closely is where that employer is established and under which country's law they're employing you, because both the withholding they apply and the social security system you fall under depend on that.
Self-employed work
If you invoice as self-employed or through a company, you take on everything yourself: registering under the relevant scheme, periodic filings, correct invoicing, managing indirect taxes where they apply, and contributions. The usual trade-off is a higher gross rate, but that difference isn't profit: it covers obligations that, in the other scenario, the company would take on.
There's a nuance that often gets overlooked. Invoicing doesn't automatically make you self-employed in the eyes of the authorities if the real relationship is one of dependence and subordination. Conversely, being self-employed doesn't remove the obligations of the country where the service is provided. The full comparison between both arrangements, with their practical implications, is covered in the article on self-employed versus employed.
Documentation is your only defence
If anything distinguishes an organised offshore worker from one who ends up in trouble, it's not tax knowledge: it's the paper trail. In the event of an audit, the burden of proving where you've been and what you've earned normally falls on you, and memory proves nothing.
What to keep
Contracts and assignment letters for each project. Rotation sheets or signed schedules. Flight tickets and boarding passes. Passport stamps, bearing in mind some routes don't get stamped and you'll need to find other evidence. Payslips or invoices issued. Bank records of payments received. Withholding certificates issued by the payer. Tax residency certificates if your advisor tells you to request them.
How to keep it
Digitised, with a folder per year and a subfolder per posting, and file names that include the date. If you also keep a simple spreadsheet with the departure and return dates for each rotation, reconstructing a full tax year goes from a job of days to a job of minutes. Anyone who keeps their rotation calendar well organised usually already has half of this work done.
Social security: a different question
A common mistake is treating taxes and social security as the same matter. They're independent systems, with their own rules and different coordination arrangements. It's entirely possible to have tax obligations in one country while being covered by the social security contribution system of another.
This isn't an administrative detail. Your healthcare cover, your protection in the event of temporary or permanent incapacity, and the pension rights you build up all depend on it. For a trade with the physical risk of commercial diving, incapacity cover is not a minor matter, and it's worth reviewing alongside the insurance and social protection offered by the contract, which are almost never enough on their own.
Errores frecuentes
The first, and the most expensive one, is getting your information in the ship's mess. Crewmates speak from their own situation, with their own country of residence and their own type of contract, and that situation may look nothing like yours.
The second is confusing not paying with not filing. In many cases there's an obligation to submit a return even if the final result is nil, and failing to meet that formal obligation has its own consequences.
The third is assuming that if the company has withheld something, everything's already sorted. A withholding is a payment on account, not a final settlement.
The fourth is changing residence on paper without really changing it. Registering as living somewhere you don't actually live is a well-known path with a predictable ending.
The fifth is not reviewing the situation when circumstances change. A change of contractor, area of operation or family situation can alter the whole picture, and what was correct one year may no longer be the next.
How to choose an advisor
Look for someone with real experience in international personal tax and, if possible, with posted or seagoing workers. A generalist advisor who's never seen an offshore contract will take longer and get it right less often. It's a consultation you pay for, and it's money well spent compared with the cost of a back-tax settlement. In the industry's forums and groups community you'll often find recommendations for professionals who already know this type of case.
How this fits into deciding where to work
One last practical note. When you compare offers, the gross figure isn't comparable on its own without knowing what's behind it. Two identical rates can end up as very different net amounts depending on the country, the type of contract and who covers travel, accommodation and contributions.
That's why it's worth always asking the same things before signing: under which country's law you're being hired, who the payer is and where they're established, whether there's withholding at source, what expenses the company covers, and which social security system you'll fall under. It's information a serious agency gives without hesitation. In the guides on pay and job markets you can see how conditions vary between regions, but what it means for your pocket depends on your personal situation.
In closing
Offshore work taxation isn't mysterious, it's just more branched-out than that of a job ashore. It's organised around three ideas: where you're tax resident, what the applicable treaty says if two countries are involved, and under what legal form you provide the service. With those three answers clear, and with documentation filed from day one, most problems are avoided before they arise.
We'll repeat what opened this article, because it's the most important thing you can take away: this is general information and does not constitute tax advice. Every case depends on the country of residence and the specific contract, and the only responsible way to resolve it is to sit down with a tax advisor, show them your paperwork, and ask about your situation, not your cabin mate's.
Frequently asked questions
Does this article count as tax advice?
No. It's general, educational information on the concepts involved in offshore worker taxation. Each situation depends on the country of residence, the contract and where the service is provided, so you need to consult a tax advisor before making decisions.
Why is tax residency always the topic?
Because it's the concept that determines which country has the right to tax your income, and to what extent. Almost every question an offshore worker has gets resolved once you first clarify where they're tax resident. It's a concept belonging to tax law and doesn't necessarily match nationality or where you live.
Does being at sea count as being out of the country?
It depends on the applicable rules and on where the unit is operating, and it isn't always treated the same way. It's exactly the kind of question to bring to an advisor with your contract and rotation schedule in front of them, rather than assuming the answer that circulates among crewmates.
Is social security decided the same way as taxes?
No. They're separate systems, with their own rules, and they can diverge: it's possible to contribute in one country and have tax obligations in another. It's worth treating them as two separate questions and checking healthcare and incapacity cover separately.
What documentation is worth keeping?
Contracts and assignment letters, rotation sheets, flight tickets and boarding passes, passport stamps, payslips or invoices, and any withholding certificate issued by the payer. Keeping it organised from your very first posting saves a lot of work and grief later on.
Published 4 September 2026 · By Arthur More, professional saturation diver
Photo: Ken Lund · CC BY-SA via Openverse