An interesting summary of the requirements (and deficiencies in the requirements) for overwater flight:
Overwater helicopter operations are permitted in the knowledge that emergency situations may arise which require an immediate and forced landing. Accordingly, ICAO Annex 6 Part 3 paragraphs 2.2.11 and 4.5.1, and national operating rules specify those circumstances where approved flotation and safety equipment must be carried, and ICAO states that Sea State shall be an integral part of ditching information.
Currently, in both JAR and FAR rotorcraft airworthiness requirements, there are two standards of flotation equipment which can be approved. The first, which is applicable to both JAR/FAR 27 and 29 is Ditching Equipment. The second, which is only applicable to JAR/FAR 27 is referred to as Emergency Flotation Equipment. The airworthiness requirements applicable to both are generally the same except that Emergency Flotation Equipment is not required to meet any prescribed standards for the water entry phase.
FAA Advisory Circulars AC 27-1 and 29-2 define Ditching as an emergency landing on the water, deliberately executed, with the intent of abandoning the rotorcraft as soon as practical. For the purposes of this paper, it is assumed that Emergency Flotation Equipment is intended to achieve the same aim as Ditching Equipment.
Following a ditching onto the surface of the water however, there are conditions which will cause a helicopter floating upright to capsize before abandonment has been completed. In these circumstances, evacuation will have to be carried out from an inverted and flooded cabin with the occupants probably suffering from severe disorientation and cold water shock. Service experience has brought about a number of design features which can significantly enhance occupant survivability in these circumstances. A number of these features are currently not required by the airworthiness codes.
Accident data indicates that unintentional water impact is also to be expected and, in otherwise survivable crashes, the major cause of fatalities is drowning. There are currently no crashworthiness certification requirements specific to water impact. Recent research into accident data has concluded that improvements in the crashworthiness of flotation equipment would significantly enhance post crash survivability.