Before we bought the camp we would spend the day on out on the lake and faced the same problem. I was crazy enough to have Mac mark a stick on the mainland and took it with us on the boat, hiding it until he needed a break. I thought that I could get him to whiz on the stick while we were out and was sure that I would be able to catch it in a bucket.
He was all for using the stick when it was in the yard, but he didn’t seem to keen on the idea when we were out on the swim platform. He really likes to snap at the water and of course he would get full. Unfortunately he had no choice and had an accident, which really upset the poor little guy. After that we were much more careful not to let him drink too much while we were swimming off the boat.