I trained my schipperke to do her business on a piece of astroturf. It worked out really well. I'd keep it on the foredeck with a grommet, and the whole thing was attached to a stanchion.
When she pees, I dump a bucket of sea water
on the astroturf, and that rinses the urine away (she's a 16lb dog; lot a lot of stuff comes out of her).
When she poops, you just dump the whole thing in the water
. That part might raise offence in some places, because that's kinda-sorta dumping fecal matter, although the seals
are crapping all day long, so you could argue that I suppose.
Just keep a piece of astroturf in the area she normally does her thing in the yard or whatever, and get her to learn it's okay to use that. It might take a while, but she'll be a lot less stressed.
Then have a different piece of astroturf for the boat. It works well for me, but I did train her that way as a puppy. Also, she pees on every piece of astroturf she sees, and you'd be suprised how much their is in the civilized world.