For women, a tankini would be better than a one-piece. For myself, I wear a bandeau (tank top) over my bag. Others use a Stealth belt, and others use nothing. It might depend on whether you are just splashing around or doing laps.
The best thing to do is take a friend (who knows about your surgery) and spend the day bathing suit hunting. I am short, chubby, and have big boobs, so it's not only about the ostomy for me. When I was younger and thinner... I went shopping by myself and picked out a very slimming black bathing suit with ruching in the front. I like it so much that I bought it in red too. The black one was perfect... the red was see-through when wet. It could happen to anyone.
As long as the suit fits well around the lower belly (supports the pouch), you shouldn't have any trouble in the water. I used to dive off diving boards or the side of the pool, and I've also spent much of my time trudging in and out of the ocean waves. Now that can pull you down and pull off your bathing suit. Never happened!
Don't put too much thought into it or blow "what might happen" out of proportion. When I get out of the water and sit down, I cover myself with a towel and just make sure everything is still intact. Except for the Dead Sea (in Israel - where my pouch fell off), I've never had a problem.
I do one of the following two things as far as a bathing suit but always wear a Sure Seal over my wafer to keep it from getting too awfully wet ... but as far as bathing suits --
Either wear a wrap or buy the bathing suit "shorts" -- both are available on Ostomy Secrets and secure the pouch. I have never even had a pouch come loose because of the Sure Seal ... I would imagine the strips you can put around the outside of the wafer would do the same thing but I prefer the full coverage of the Sure Seal.
I wear dentures and my bottom gums are raw and painful from the Crohn's. Also, the skin all around my vagina area is raw and bleeding. It burns when I pee. I am ready to throw in the towel. So tired of this. It's been months. Any help or advice, please!