I don't see why in the world you would want a dog to go in a litter box! Good luck teaching him to cover it up like a cat!
The worst part of a cat onboard is that they track their litter from their paws all over the cabin
of your boat. I always thought one of the great advantages of a dog was that they'll do their business on the foredeck, and you can easily clean it up.
I have friends whose big dog always used the catamaran's trampoline. Now, that's the cat's meow!