I'd bring that roll of TP along in your backpack just in case. I've spent hours and hours on my brother in law's boat and never needed to go so it's possible to hold it. Go light on your breakfast before you leave to go and try to have a light lunch as well. Have fun! 

If you can get a prescription for Lomotil it might be worth seeing if that works better for you than Imodium. Try it out before the trip, though!

Have fun!

I would skip lunch, drink a lot of fluid and have a bigger dinner when you get home.

If I were you I would take TP and nothing else. I think you will be surprised at your j-pouch while rafting (unless you have pouchitis or something). When I'm in Colorado hiking and such, I typically have to go less than when I'm at home. I really think it's due to the distractions of the day + the dry air that dehydrates me more than at home.

I've been! I had to poop in the forest and use a leaf for TP. It was very glamorous. But I had fun and everyone laughed about me pooping because I told them my situation. People are understanding. Looking back I would have had a protein shake for breakfast, skipped lunch and ate a half a Metamucil biscuit before I headed out. Don't let this make you miss out on the fun stuff.

Hey there!  I went rafting several times with my pouch, and I had a great time.  Load up on the immodium/lomotil and stick to your "low residue staples" that move your gut slowly (peanut butter sandwiches and bananas were always my outdoor adventure "go to").  Put some TP or wet wipes in your bag as well as some antibacterial hand gel.  Make sure you hit the bathroom right before you leave, even if you don't have to go.  Finally, don't let yourself worry about what would happen if you have to go in the woods.  Most people will likely have to answer the "call of nature" at some point on a day trip, and a minute or two of waddling around in the forest is well worth a day of fun.  Have a great trip!