The SAS - Boat Troop
Often confused with the Royal Marines Special Boat Squadron (SBS), Boat Troop carry out the SAS's activities that require a water insertion platform. For example possible mission scenarios are, the re-taking of an ocean oil platform, planting explosives on a ship, underwater insertion via submarine, patrolling in-land canals (which I think they did alongside Navy Seals in Columbia), or the capture of a ship or tanker.
They are rumoured to be in operation in Afghanistan alongside the SBS and are known to have been the troop used in the re-capture of South Georgia (along with SBS and regular commandos) during the Falklands conflict.
They use all sorts of infiltration methods, ranging from canoeing in land (as they did in the Falklands) to insertion by Rigid Raider.
The most important thing for a member of Boat Troop is to keep a cool head, remembering that they conduct all the same missions as other troops, such as planting explosives, but they often have to do it, at night, whilst underwater in freezing cold and pitch black conditions.