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 bathingsuity wish ruching in the front. I like it so much, that I bought it in red too. The black one was perfect....the red was a 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 use 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 migh happen" out of proportion. When I get out of the water, and sit down, I cover myself with a towel, and just make sure everthing is still intact. Except for the Dead sea ( in Israel - where my pouch fell off) I've never had a problem.
Best of luck to you.