Hydrophones having any of the following: (a) Incorporating continuous flexible sensing elements (b) Incorporating flexible assemblies of discrete sensing elements with either a diameter or length less than 20 mm and with a separation between elements of less than 20 mm; (c) Having any of the following sensing elements: 1. Optical fibres; 2. ‘Piezoelectric polymer’ films other than polyvinylidene-fluoride (PVDF) and its co-polymers {P(VDF-TrFE) and P(VDF-TFE)}; 3. ‘Flexible piezoelectric composites’ 4. Lead-magnesium-niobate/lead-titanate (i.e., Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 , or PMN-PT) piezoelectric single crystals grown from solid solution; or 5. Lead-indium-niobate/lead-magnesium niobate/lead-titanate (i.e., Pb(In 1/2 Nb 1/2 )O 3 –Pb(Mg 1/3 Nb 2/3 )O 3 –PbTiO 3 , or PIN-PMN-PT) piezoelectric single crystals grown from solid solution; (d) Designed to operate at depths exceeding 35 m with acceleration compensation; or (e) Designed for operation at depths exceeding 1 000 m. Note : The status of hydrophones specially designed for other equipment is determined by the status of the other equipment.