I just saw this product advertised on TV. I don't know anything about it other than what they said and what is on their web site, but maybe it will work for your situation.

https://pottypatch.com/

One nice thing is that if you can get your dog to use this at home, maybe the transition to the boat will be easier.

Good luck.

Bud

+ 1 for the sod. My friend has a mini pincher who uses a piece of home depot sod in a low plastic bin. My cattle dog has went over and used it a few times.If you can water it twice a week, it lasts a long time. Tommy