I've been trying things during this first year with my ileostomy (dec 09) - for swimming as well as hot-tubbing
Like Healthyguy, I'm using the Convetec 2 piece system with the sur fit flexible flange.
Without anything extra, the hot-tubbing doesn't work. After 15min, the adhesive (the 'tape' part, not the flange) begins to melt and gravity draws the melted adhesive downwards and it starts to come out the bottom edge. If I'm just swimming, the seal holds pretty well, but I find I'm changing every other day (if I'm in the water many times a day or daily). Once melted, the adhesive never really solidified/dries again and it's very irritating.
Then I tried adding Perma-type Security Strips http://www.perma-type.com/accesil.htm
tape ... a bit better for swimming, but still a no-go for the hot tub. The flange 'tape' still melts.
Now I'm using Sure Seal Rings http://www.bestbuyostomy.com/product/ACT%20RS%200110
Better but still get the melting in the hot tub, and I think that's just the way it's going to be.
Have to admit, I didn't enjoy my first summer heat like I used to. Itchy, sweaty. The answer was to change my appliance more often.
I've decided that hot tubbing may not be something I do as often as I did before. But I'm going to keep at the swimming ... we're going to Hawaii for our 25th in Jan and we love snorkelling. I'll probably be using my support belt for additional comfort (of body and mind).
Hope you find something that works.