Because the alternatives fail. Electric tools are unacceptable at depth, and pneumatic tools lose power rapidly as depth increases since the exhaust must work against water pressure. A closed hydraulic circuit returns fluid to the power pack rather than venting, so ambient pressure does not affect it.

No, and that is the format's defining advantage. Because the fluid is returned through a hose rather than exhausted to the surroundings, the circuit is unaffected by ambient pressure and a tool performs the same at depth as at the surface.

Materials selected for seawater corrosion, seals designed to keep water out rather than only oil in, immersion-rated and often buoyancy-adjusted hoses, and controls that can be worked in gloves with poor visibility. The hydraulics are conventional; the packaging is not.

Environmentally acceptable biodegradable fluid, which is widely required for subsea work and is frequently a condition of the permit, because a hose failure releases fluid directly into the water. Establish the requirement before mobilising rather than discovering it on site.

On the vessel or the quay, with only the tool and its hoses entering the water. Hose length therefore becomes a real design consideration, since pressure drop along a long run reduces tool performance and has to be allowed for when sizing the pack.

Grinders and cut-off saws, drills, impact wrenches, chain saws, chipping hammers and pumps, among others - broadly the same range as surface hydraulic tooling, built for immersion. Specialist subsea equipment such as dredges and cutting tools extends the range further.

Rinsed thoroughly in fresh water and dried, since salt left on a tool attacks it between jobs far more effectively than the work does. Seal condition should be checked regularly, because a seal that admits water contaminates the fluid and the damage runs back to the power pack.