OMG, I know people who don't have IBS, and who were mortified at the prospect of spending hours on a pontoon boat! Yikes!
That said, here's what I do when I know I'll be away for hours and I've been suffering with bouts of D: I constipate myself by taking either two Imodium, or two Saccharomyces boulardii capsules. That has always stopped the diarrhea immediately and for the rest of the day, usually stretching well into the next day or even the day after that. Why didn't I take just one pill? Tried it and it didn't work at that dosage, so I doubled it. Mission accomplished. Isn't that always the way? So if you haven't already, you might try constipating yourself to get through the day. Then deal with the aftermath later when you have alone time.
That's all I got. Let's hope others will eventually chime in too. I wish you oodles of luck with finding what works 😀