I've struggled with this too, especially because I'm (ha ha, WAS) a competitive swimmer. I needed something for laps. They do sell maternity suits suitable for lap swimming at swimoutlet.com. I ended up going to the rack with last year's styles at TJMaxx and buying the largest size they had (18?) for $16. It's not ideal, but I'm still doing laps at 38 weeks in it!

I realize this probably doesn't help your inquiry, since racing suits aren't cute, and at 38 weeks, the chest coverage isn't that great because the belly bulge pulls the material down a little too far for my taste. But for $16 I'm working it!

I won't be pregnant for much longer, so thankfully I haven't needed a swimsuit. I went to the Caribbean during my last pregnancy, though, so I ordered one or two maternity tops and bottoms from Target (I think they were different brands, too - I think the bottoms were Liz Lange and the tops were something else, like Merona), and I bought a very big tankini top at JCPenney for something like $10. They worked fine, but I was only about 5 months along at that point. The next summer I was able to swing them while I was losing weight - the maternity tops were clearly funky for me, but the regular, bigger tankini top was ok, and the maternity bottoms were really just big bottoms

I don't know how much support you need, but if you need a lot, I second the sports bra idea, and I will say that a lot of times you can get away with just buying a bigger suit. Like TheLaser said, one pieces can start to pull when you have a belly, meaning you are showing a lot of cleavage, but you should do fine if you go for larger (non-maternity) tankinis. Most of the time the maternity ones aren't flattering and/or are overpriced.

I found I could use my non maternity bathing suit until about 30 weeks (I wear hipster style bikinis). I did buy a backup at target which worked fine;
http://www.target.com/p/women-s-maternity-twist-hipster-swim-bottom-black/-/A-14205281

Cute! Glad you found one!