I tend to do my underarms from the inside of my biceps all the way down to mid ribs...rub your arm along your torso in a swimming motion and you'll see what I mean. I also tend to do my neck area where the wetsuit will be in contact.

I also apply it to the crotch area but that's more for the bike and run than the swim. I just don't want to do it in T1 (unless Half IM or longer).

Some people do their wrists and ankles but I don't.