I think it more heat then wet that causes leaks. If it looks good before you go in you should be OK.
I agree with xnine - I am swimming a lot right now, and I don't have issues with leaks. What I do find is that the filter on the bag doesn't seem to last as long, even though I put the stickie over it before I go in the pool. Generally, I find that summer is hard on ostomy appliances; whether from the heat, or swimming.
Living in Florida- between pool and ocean, no problem with leaks. I do wear a wrap by ostomy secrets for comfort while swimming. Stay hydrated and enjoy. 😎
I don't swim that much but when I do I have no troubles. I have a colostomy. Best to you.