To be fair personnel working offshore do not haver a choice in their clothing when flying. It is mandated by the companies. argue with the requirement - no job!
However it is all based upon a goodly amount of thinking and is part of a system which is supposed to give 'a good prospsect' of recovery to a place of safety.
The general criteria is that personnel must be able to survive for up to 2 hours in the sea. Hence the use of dry suites with thermal insulation garment underneath. If the air temp is warm then yes you sweat like a pig, but if you end up in the water the helicopter has an EPIRB, the pilots have EPIRBS and transceivers and increasing you will have a PLB (baby EPRIB). That with the amount of shipping and Coastguard aircraft should mean that an enroute or destination ditching should not result in floating bodies.
The use of rebreather is also near universal but that is largely driven by the poor access/egress that most large chopper 'offer'