Underwater, tools aren't electric: they're hydraulic, powered from a power pack on the surface or on the vessel itself. Grinders, impact wrenches, saws, drills, hammers and water jetting equipment cover almost all the work. The main risk isn't the tool itself, but reaction torque and the lack of a firm point of purchase.
A good part of a commercial diver's job is doing underwater the same things an operator does on land: cutting, tightening, drilling, cleaning, grinding down. The difference lies in the power source, in how you're secured, and in the fact that any mistake has a much narrower margin for recovery.
Why hydraulic
The reason is simple: safety and reliability. An electric motor underwater demands flawless insulation and adds a shock risk that nobody wants to take on with a diver in the circuit. Hydraulics, by contrast, work with pressurised oil in a closed circuit, tolerate immersion well and deliver plenty of torque in relatively compact tools.
The setup is always the same. A hydraulic power pack — the power pack— sited on deck or on the installation generates flow and pressure. From there, two hoses run down, a pressure line and a return line, to the tool the diver is handling. That tool umbilical is separate from the diving umbilical, which carries gas, communications and hot water, and keeping it managed and tidy on the bottom is part of the job.
Pneumatic alternatives exist for some tasks, powered by compressed air, but they lose efficiency with depth and generate a considerable volume of bubbles that cuts visibility. That's why hydraulics dominate.
The tools in use
Grinders and wire brushes
Used for surface preparation: stripping coatings, rust and marine growth before an inspection, a weld or a thickness measurement. It's an unglamorous task and an extremely common one, because almost no serious inspection is done on dirty metal.
Impact wrenches
For fitting and removing bolted joints: flanges, clamps, anchors, structural elements. On subsea jobs where a flange has spent years in the water, the torque needed to break the corrosion is high, and the reaction on the operator grows with it.
Saws and cutting discs
Band saws, reciprocating saws and cutting discs let you cut steel, concrete or rope without resorting to thermal processes. It's the preferred option when there's a risk of trapped flammable gas, which is common when working inside enclosed structures or near tanks.
Drills and core drills
For drilling anchor holes, taking core samples or fitting fixings. They usually work with a magnetic base or a frame fixed to the structure, precisely because drilling isn't viable without support.
Hydraulic hammers and chisels
Used for concrete demolition, stripping thick coatings and submerged civil works, from quays to dam gates.
High-pressure water jetting
El water jetting cleans surfaces with water at very high pressure, with or without abrasive, and in powerful versions it can even cut metal. It's one of the most effective tools in the trade and also one of the most dangerous: a jet at that pressure will go through a suit and cause serious internal injuries that don't always look serious at first. It's worked with dead-man triggers, strict procedures and a safe standoff distance.
Dredging and suction
Suction pumps and eductors remove sediment to expose a buried structure. Their characteristic risk is entrapment: a suction hose that grabs a glove, a suit or a hand needs an immediate response and a valve that can be shut off from the surface.
Cavitation
Cavitation cleaning systems use the controlled implosion of microbubbles to lift off marine growth without damaging the coating. They're used mainly for hull cleaning and on painted surfaces that can't take abrasion.
Reaction torque: the risk that defines the trade
On land, when you tighten a bolt with an impact wrench, the ground and your body weight absorb the reaction. Underwater there's neither ground nor weight: the diver is practically neutrally buoyant. If the tool spins hard, what spins is the diver.
That gives you the first practical rule of tool work: before triggering anything, you need to be firmly secured to the structure. Harnesses, positioning lines, platforms and frames are used. A diver holding a grinder while holding himself in place with the other hand isn't working, he's about to have an accident.
The second characteristic risk is entanglement. Diving umbilical, tool umbilical, tag lines and hoses all share a confined space, often with zero visibility. The discipline of keeping lines tidy and separated is as important as tool technique itself.
The third is stored energy. A pressurised hydraulic hose that fails releases oil at high pressure; a hose that comes loose turns into a dangerous object. That's why depressurising before connecting or disconnecting isn't just a formality.
Maintenance and control
Maintenance of the hydraulic set-up is done on the surface and is part of the project's management system. The usual checks are hoses and fittings, leak checks, the condition of the fluid, cleaning couplings before connecting them, and verifying that the tools have been tested before going down.
One detail that surprises people coming from another trade is how much the fluid matters. On operations near sensitive areas, biodegradable fluids are used, because a leak into the sea is an environmental incident with contractual consequences, not just a technical hiccup.
All the tools that go down are secured with tag lines. The immediate reason is not to lose them; the deeper reason is that a heavy object loose in the working water column is a risk to everyone.
Where this fits into a career
Tool handling is what separates a diver from a mere swimmer in a helmet. Formal training practises it from the start, and on your first contracts most of the learning happens exactly there: securing yourself properly, keeping lines tidy, communicating before triggering, knowing when to stop.
It's also a mark of professional value. Hours logged with a specific technique carry weight when a contracting company is looking for specific profiles, which is why it's worth logging them in detail in your logbook. If you're starting out, in how to get into commercial diving with no experience and in how to become a commercial diver you'll find the full path, and in equipment the rest of the material that goes alongside these tools.
In the end, underwater tools aren't any harder to use than the ones on land. What's hard is the environment: cold, current, poor visibility, limited time and a surface team that can only hear your voice. Everything done well with a tool underwater is done slowly, secured, and with a call-out beforehand.
How a tool task is set up
Before the tool goes into the water there's a sequence that's rarely skipped on a well-run operation. The task and the sequence of movements are defined, the tool is chosen and tested on deck, hoses and couplings are checked, tag lines are rigged, and it's agreed how pressure will be called for and how it will be cut.
That agreement over pressure matters more than it sounds. The diver doesn't trigger anything until confirming they're secured, and the surface doesn't send pressure until hearing that confirmation. Cutting it must be immediate and possible from above, without depending on the diver being able to speak.
After the task there's the less glamorous work: recovering the tool secured, bringing up hoses without tangling them, washing down with fresh water, checking and logging any fault. A tool that comes back to deck in poor shape is the one that fails on the next dive.
Frequently asked questions
Why are underwater tools hydraulic rather than electric?
Because hydraulics work well underwater and without an electrical risk to the diver. A closed hydraulic circuit tolerates pressure and water far better than an electric motor would, which would demand perfect sealing and add a shock risk that's hard to accept.
What is reaction torque, and why is it dangerous?
It's the force the tool sends back to the operator as it spins. On land it's absorbed by your feet and body weight; underwater, without a firm point of purchase, it's the diver who spins. That's why the job starts by securing yourself to the structure.
Can steel be cut underwater without electricity?
Yes. Besides oxy-arc cutting, there are hydraulic saws, cutting discs and abrasive water jetting equipment that cut without a flame or an electric arc, which is especially valuable when there's a risk of a flammable atmosphere.
Who controls the power pack during the dive?
The surface team. The power pack and its hoses are part of the set-up handled from above, coordinated with the supervisor, and the pressure is cut from there if the diver asks for it or if a problem comes up.
Do you need specific training for these tools?
Basic training is included in recognised commercial diving courses, but many specific tasks require additional endorsements or qualifications, especially in cutting, welding and work with high-pressure equipment.
How do you avoid losing a tool on the bottom?
With tag lines and safety hooks. Every tool that goes down is secured, both so it isn't lost and so it doesn't fall on anyone or on the structure while being handled.
Published 23 August 2026 · By Arthur More, professional saturation diver
Photo: ell brown · CC BY-SA via Openverse