Re: Boating with a toddler..Where to potty?
boys pee over the side, or into a P-cup (a well-marked solo cup). Girls pee into a bucket and dump it. Older girls know how to hang over the ski ladder. Boys are taught at a young age to be gentlemen and to turn their heads. Police any mirrors and reflective surfaces.
No need for a porta potty (do you carry one backpacking?).
If you feel like you have to save it to dump on shore, I heard a great suggestion here once--put a disposable diaper in the bottom of the garbage bag to absorb and prevent leaks and spills.
I remember fishing with my dad when I was about six, in a 14' whaler; he told me to go over the side, I said the other boats might see, he said, "if they haven't seen it by now it's time they did."
P-cups are also a very smart safety device for men on larger boats fishing solo and/or in cold weather, or men who can't swim or climb back into a boat unassisted (most of them). Many drowning victims are found with open flies.