I haven't tried the freshette thing in a kayak, but I have used it on a small sailboat, and directed the pee into a plastic bottle to get rid of later. It's pretty slim, and when you get used to using it (a lot of it is the mental thing of relaxing the bladder muscles when you feel like you shouldn't), it's easy. It is kind of pricy, I guess, but it lasts forever. I have had mine for over 10 years now.