The purpose of the in-water survey is to obtain information on the condition of the hull and machinery of large vessels normally obtained from the Docking Survey. The survey may be carried out instead of any one of the two docking surveys, under the surveillance of a surveyor with the ship at a stable draught in sheltered waters, the in-water visibility is to be good and the underwater hull clean.

The survey requires a self-propelled, steerable survey vehicle fitted with a long-range TV camera to aid steering and check for hull distortion also a close-up high-resolution TV color camera to give a true picture of the state of coatings and for inspection of weld seams. In some cases a 35mm still camera is fitted. An ultrasonic probe is provided to measure plate thicknesses and other equipment includes a depth meter and speed indicator. Power is supplied and information is relayed using an umbilical from the vehicle to the survey boat

The survey boat usually houses a console containing TV monitors. plate thickness printout, audio cassette recorder, video recorder and playback unit, diver communication system, vehicle control system and associated instrumentation.

The survey vehicle is taken to the starting datum by a diver. With the aid of one of the TV monitors and using the shell expansion plan as a map, the vehicle may be guided, from the control console, over the bottom and sides of the hull by following weld runs and by reference to other features such as inlets and tank plugs. Pictures and navigational information are relayed back and video recorded along with plate thicknesses giving the surveyor an integrated visual record of all relevant information. In addition, a plate thickness print-out can be produced and/or an audio recording. The vehicle will also provide pictures of such items as the stern frame, rudder, propeller, bilge keels and hull openings although a diver may be used with a handheld camera for closer inspection of these items and also for inspection of plating on the turn of the bilge. Divers are used to measure stern bearing wear down and pintle clearances, and to inspect such things as stern seals, anodes, pintles and rudder stock couplings.

To facilitate underwater surveys plans must be submitted showing the external features of the hull below the sheer strake together with a key plan indicating the location of these features, also of reference points and the position of water-tight and oil-tight bulkheads. Notes are included on the proposed methods of marking and identifying plates. To assist divers color photographs should be provided of items such as shell openings, rudder closing plates and wear-down gauge plugs.

Provision should be made on the ship identifying bulkheads and frames above the waterline and also for establishing the identity and position of each propeller blade from inside the ship. The design of the ship must facilitate water inspection and repair, for example, sea inlets must be capable of being blanked off and drained to bilges, shell gratings hinged if practicable and anodes easily changed.