I have large breasts (currently I am busting out of my 34G at 35w pregnant, sigh) and being totally naked is uncomfortable to me for that reason. In my past 2 births I did not care AT ALL about being naked otherwise, and I am not normally an exhibitionist (though not super modest either). I just kept on my regular bra and had a nursing bra handy for afterwards. I was in and out of water and don't recall really caring about being wet or anything. Good point about bikini top though- I bought a bra-size bikini for the summer with excellent support so I think I'll put that in the waterbirth kit :)
Nice to have some extra hair clips/ties around if you have long hair, and remind your partner/helper to keep after your glasses if you wear those. With my first I lost track of my glasses in the end of labor and then I was like "I can't see!!" after baby was born. Not cool!
You might want to check on the possibility of warming up the room/home where you will be in case its cool weather- the newborn will like to be warm and that way you won't be chilly from being wet and naked afterwards.