Is it a puppy?...
You have already got some good suggestions, but here are some additional ideas...If you have a puppy, you can incorporate potty training on the boat as well as at home. Crate training will work well in both places. Get a small foldable crate to bring to the boat while still a puppy. Locate the crate in a place that is close to where you are so the dog doesn't feel isolated.I beleive in positive only reinforcement for training and very much beleive in the clicker training methods; the clicker becomes optional pretty quick, in fact not really needed at all. Use a quick word like "YES" in a happy voice to mark the desired behaviour. Follow up with a treat (My dog is really fond of cubed chicken breast).Here is what to do with a puppy. Crate train...Dog goes in crate, comes out after the number of hours that equals its age in months (up to about 5 or 6 months). 2 months old, should last two hours. As soon as the pup comes out of crate, he goes to designated potty place and gets no play time until after he releives himself. As soon as pup finished its business, click (or say "YES") and follow up with a good treat and play time. After being out for 1/2 to 1.5 hours, pup goes back in crate to start the cycle again. If pup doesn't releive himself, back in crate, no play time or extra attention.When the pup starts to get the idea, use a marking word (Potty, Busy, etc.) and use that when the pup starts to go, then "YES" and treat after. Soon the pup will associate the command. This should work well for a grown dog, too, but you need a much bigger crate! My dog will pee on command, which is very helpful on cold snowy nights when I want to get back inside!To mark a place on your boat, you might try putting a piece of carpet or astroturf in the regular designated potty spot at home, then bring that to the boat to help pup make the association.Our trainer has many clients teach their dog to ring a bell when they need to go, and it is quite amazing! We opted for whining instead (dog will never lose its whiner, and I refuse to jump when a dog rings a bell) and she will whine when it is time to go out.Good luck and fair winds...Tom