Marie13 was talking about wearing a sports bra under a drysuit. I'm pretty sure it would end up chafing under a wetsuit.Though it is nice to hear women wear sports bra under their diving suits. I never had anyone to ask what I should wear under it if I started diving so I was debating just going topless under it.
As for going topless under a wetsuit... Normally you take the wetsuit off, or at least down to the waist, between dives. Neoprene is awful for temperature control on the surface. If it's warm, you'll bake and if it's cold you'll freeze, especially if there's airflow such as you get in a moving boat.