I had never thought of this until I saw a lifeguard at Jane's water exploration class wearing a neon yellow sports bra under her red lifeguard suit. It looked cute, but also was an "Aha!" moment. Bathing suits that fit me never have enough breast support. And I knew if I was going to try the aqua aerobics I would need it!
When I workout in our sun room I double up on sports bras, and I still sometimes have too much bounce for comfort. In the water one sports bra and bathing suit plus the water has proven to be enough for me to be comfortable and not worry about wardrobe malfunctions or nip-slips. The sports bra does smell chlorine-y like a swimsuit, although I'm sure a few washes will cure that (or I can just have water-only bras if I cared about how I smell).
From now on every time I swim or play in the water I'll be sporting a sports bra underneath.