If you're alone and can't stop and don't have anything the other commenters mentioned and your boat is fiberglass and in good condition? Just pee inboard on the

If you're alone and can't stop and don't have anything the other commenters mentioned and your boat is fiberglass and in good condition? Just pee inboard on the deck in a place with good drainage and wash it off. Can't be much worse than the droppings that birds provide. But, personally in general (car camping especially) I like the humble pee bottle. At Walmart in the RV section they have the blue chemicals used in portable toilets and plane lavatories and the like. You can get it in a little tide pod format. Cuts the smell and keeps things sterile. Drop that into any container (preferably non-translucent and 2+ liters) add a little water and you've got your own miniature portable toilet. You can also buy a real portable toilet, they are quite nice these days, but can be bulky. The keyword to find them is "cartridge toilet" because typically the waste volume is a container that is detached and carried to the dump point. I once hiked 3 miles holding one of these portable toilets just so I could take a dramatically scenic (and civilized) poop and it was absolutely worth it.