I hear this question a lot : "what do people wear in the birth pool?" Here are some ideas that might be helpful and that you may already have in your maternity wardrobe.
1. Tankini top. Bathing suits are made for getting wet, so this is likely to be a comfortable option and one that you may already have, especially if you've been pregnant in the summer. Target has some cute and affordable options.
2. Camisole top. These are such a great layering piece and are relatively easy to find. Some great places for these are Target, Motherhood Maternity, and Old Navy. Go for black or a dark color so that it's still fairly opaque when it gets wet.
3. Sports-style bra. I had one that zipped in the front that I wore when I was in labor with my middle son, so it was easy to take off when I got out of the pool and put on dry clothes. I got another one (exactly like the one pictured below) at Motherhood Maternity, and I have seen similar ones at Target. Wal-Mart always has the really inexpensive, soft stretchy ones that pull on over your head; those will work well for gentle support after baby's born, too.
4. Nothing. You don't have to wear anything at all. Do what feels best for you!
When I was preparing for my births, one of the things that I did was set out camisoles and sports bras along with my birth supplies. My husband and doula were able to find what I'd planned to wear in the pool without having to look around for them, and later when I decided to get out of the pool for a while I had several sets of dry clothes ready to wear.