I wouldn't say Open water swimmers are in optimum shape.
 It can be REALLY cold- a little padding definatly helps out!
 My skinny frined was DYING. They told her to strip down and get warmed up in the bottom of the boat.

They were all pretty excited that a AZ girl swam with out a wetsuit.

I think its because my swim lessons in April and all my Ice baths in the winter helped me adjust to cold water temperature.
I'm also not stressing to much about a little bit of insulation...really came in handy!