It appears you went offline without seeing my request for information. I don't want you to have to wait any longer, so I'm going to go ahead and answer without knowing those things.
Dogs generally cannot hold their urine or feces long enough to cause actual health problems, but doing so can be very uncomfortable, and for a well house-trained dog, cause a lot of emotional distress. Your dog would eventually go on the boat, but would feel he had done something wrong.
The first thing I recommend is teaching him a 'go potty' (or whatever word you prefer to use) command while he is on land. Have extra tasty treats in your pocket - I like to use bits of cheese or hot dog. Take him outside and say 'go potty.' When he does, praise him and give him treats. Repeat this each time you take him out. Most dogs associate the words with the act after a few times. My dogs will urinate whenever I tell them to, even if they don't have to go all that badly.
Once your dog knows what the words mean, you can start teaching him that it's all right to do his business on the boat. I suggest getting the aids made for housebreaking puppies. You could place a piddle pad on the astroturf until he gets the idea. There are sprays, such as 'Go Here' you could spray directly on the astroturf. These things encourage dogs to go in the areas where they smell them. For male dogs who lift their legs to urinate, there are plastic fire hydrants or spikes, also containing the encouraging scents, that will help even more. Here is a reputable online source of all these supples:
The next step would be to place the prepared astroturf on the ground. This will feel more like outdoors to your dog than it will to immediately transfer tot he boat. Make sure he has to go by waiting until first thing in the morning. Take him to the spot and tell him to go potty. Praise him and give him treats. When he will do this willingly, it's time to transfer the whole process to the boat. Repeat the command and treats as often as necessary for him to get the idea. Once he is comfortable with using the astroturf, you can gradually discontinue using treats.
If you have more questions let me know in a REPLY. I wish you fast success with this method.
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