A number of years ago I had to do sea survival training for work, run by some ex navy types. Being somewhat top heavy, I found it really difficult to get the floatation vest to sit in a way that was much use. The instructors were really clear that it had to fit tight against the chest to be of use, but right against my chest is a very different angle to tight against a man's chest. Not wanting to draw my male colleagues attention to my massive knockers I didn't really say anything, but it was clear to me that I was having to expand more energy to keep my head above water when floating than they were.
I do think it must be very difficult for our female military, where presumably, almost everything just doesn't quite work properly.