Sarah,
I'm so excited for you and your family! It sounds like a much-needed vacation. My sister and her husband honeymooned in St. Lucia, and they loved it!
I live in Florida (where it's hot 6+ months out of the year), so I swim a lot to stay cool. My favorite bathing suit is a one piece with a skirt, because it's comfortable, flattering, and hides the pouch perfectly!
Thankfully, I've never had an issue with water compromising my wafer seal. I wear the Coloplast Assura Extended Wear Wafer (#2833) with matching cloth covered pouch (#13986) and don't need Sure Seals to prevent the wafer from peeling off. You might want to test your current wafer in the bathtub to see how it does. Since you have three weeks, you have time to prepare, in case you want to try out a different system.
Some tips are to pre-cut holes in your wafers, so you don't have to carry scissors in your hand luggage and pack all liquids (like deodorizers) in air-tight, ziploc bags in your suitcase, so they don't leak all over your clothes.
Make sure to take lots of photos and feel free to post some on our photobucket.com page! (username: crohnsdisease, password: 6mp3asa)
You're going to have a great time!
Cece