I would figure out how much you will run your engine and match your gal per hour with that. If you want 50 gal a day and you charge for two hours a day you hill need something that produce 25 gal an hour. Also, if you dont run the watermaker every few days you will have to pickel it or fresh water flush. You will also need to constantly check your prefilters. Oil, chlorine and gunk in water is BAD for a watermaker. lots of junk in a bay, but it can be done. You can get automatic ones that flush when needed.
Buy the way, How big is your holding tank. If its not a match to your water tank, you will have to go to a pump out station anyway. So just fill up then. I know you wont discharge all that waste at your mooring. Would you? Water makers are nice, But you do have to pump out, no mater how much water you have. Please keep clean wake.