I just used a size 40 nike swimsuit. My normal size swimsuit is a 34/36 depending on the brand. It worked for me all the way up until I delivered. I swam the day before my induction.
I did the same. My boobs did start to spill, so I wore a Speedo sport bikini top underneath. The chlorine or salt water in the pool will loosen the suit up, especially if you're in the water daily. Also look for a longer torso suit too, that might help.