So here are the suggestions I have received on this thread or in private email:

1. "Nothing comes between me and my wetsuit"
2. Skimpy, Speed-o style swimsuits
3. Lycra shorts, which may cut down on sweat
4. Rash guards
5. Boxer shorts
6. Old bicycling shorts - presumably the lycra ones?

In private email, someone suggested that (1) is the most common, but (3) is becoming much more popular.

Is that about right?

Jonathan