Immersion suits when required, yes. Even marginal conditions it makes sense to wear one offshore. Helmets? No way. I can understand the thought behind wearing a helmet but is the risk really that great that us offshore bods need to wear one? I don't think so. Also, performance on say an S76A+/C whatever, is acceptable to the powers that be and proper planning will give you some margin for error. 139's would be nice all round but not all companies want to spend money when they can get away with not speding a cent and keeping old a/c in the air.
The exposure time is exactly that. Reduce/minimise the exposure time and you'll have more time to fly away rather than ditch.
Helmets would be good for bird strikes, but do you really think a blade coming through the cabin would have less effect if you're wearing a helmet? I think not.
As for hitting your head in turbulence. Utter cr*p!! Tighten your seatbelt and shoulder harnesses.