Well, I wear a size 26W with 48F bust so I understand trying to find something that will support you in the water aerobics.
I bought a really nice swimsuit by Delta Burke specifically to use for water aerobics a few years back. At the time, JCPenney sold it but I think if you did a search on the internet under Delta Burke's name you might come up with something.
It cost me $80 but what I liked about it was that it was a higher neckline so there was no cleavage, it had a "cross strap" in the back for upper back and more bust support so my "girls" stayed put when I was being active in the water. It was also quite flattering. It was worth every penny/dollar I spent on it.
Good luck and you will love water aerobics. I would love to do that again. It was so much fun and you really do get a very good work out.
