The wafer I wear stays on when I swim. It's the Coloplast Assura Extended Wear wafer. I live in Florida and swim a lot, so it's been great to find a wafer that doesn't fall off when it gets wet.
Others have recommended using Glad Press n' Seal or Sure Seals (which you order with your ostomy supplies) when they shower or swim, but I've never tried them, so I don't know how well they work. Maybe you can try them out in the bathtub.
I hope you find something that works for you, so you can go swimming on the 4th!
Dx'd Crohn's in '99 at 28. Proctocolectomy and ileostomy in '06.
Disease-free, medicine-free, and very thankful to be healthy again.