Going to the bathroom on a small boat?
I agree on the five gallon pail...although in smaller boats, who wants to bring that? Folgers coffee and some other brands have those plastic coffee containers with the handle. When packing for a trip, I usually store the toilet paper right inside the container. Then on the boat, you have everything handy. It takes some gettin used to, but once you go a few times in a boat, it's not too strange anymore.
If your boat is small, like a 12' or 14', a coffee can might be a bit easier. Also a gallon milk jug with the cap in place and the bottom cut out. A container with a handle is helpful. She can rest her bottom on the seat edge and hold or place the can down below. By resting on the seat edge, it will help her keep her balance. This is what I do.
I'm glad to hear she desires to go with you! I'm sure you'll add a few more priceless memories to your bank! Good luck and have fun.