You must contact the cruise line well ahead of time. They will provide an on-board toileting area...either puppy pads or a box of dirt or turf. I assume if it is an assistance dog that it is well trained to "go" on command. Be sure that they are willing to use a box (some dogs object to this). As ashore, the dog will have full access to the ship when with the owner. The dog should not be left in the cabin alone though.