Diasagree with others
If you have accepted a deposit and the offer, then you really shouldn't be showing the boat to other potential buyers until the person with the deposit either backs away from the deal or some goes amiss during the sea trial. If someone calls about the boat, you should tell them that you have taken a deposit on the boat from a buyer, take their name and number, and ask them if the deal falls through can you recontact them.
It's like buying a house, the seller accepts your offer and you put down a deposit until the home inspection. You stop looking because you have found the house, so you may have passed on other houses that meet your criteria. However, the seller shows it after they have accepted your offer and the person looking at the house says "I want it and will pay X dollars more than what your current offer is". If you were the seller, do you take the new offer? If so, you breach the contract and the buyer can sue. Another example, you see an add for a item in the paper that is a high dollar amount (car, TV, trip, etc.) and of limited quantity, but you can't get there before the sale or store closes. You call in and put a deposit on it until you can get in and fully inspect and pay for. You arrive only to find that the item was sold to a walk in customer. On the good side, you do get your deposit back.
Although Boat Purchase agreements are not as stringent as the buyer can basically walk at anytime without penalty, the principle is still the same. Ethically, I think it is wrong.
One more thing to consider. What if you show the boat to a another potential buyer, after accepting the deposit, and he/she breaks something on it while looking? Now the boat is different to the original buyer and they have every right to walk away. Then you've lost your buyer AND you have to fix the thing the "looker" broke.