FredFri wibbled: The one I know says that for any flight to/from helidecks you
always need (in addition to the aircraft equipment)
to wear lifejackets. Survival suits are mandatory only when sea temperature is below 10°C
or at night
or surface wind speed > 35 kts."
Yeh right. In my book, wear a life jacket over water if you are too far to auto to a safe landing point [dry] and wear a goon suit with under insulation anywhere in Europe or vicinity irrespective of the sea temp, day or night. Try spending 2 days in a dinghy in the north sea in summer and you will see what I mean. Hell don't take my word for it, go talk to the sailors who end up in the drink without. [No I didn't ditch.......training.............which went slightly awry. 
]
Best Wishes