It is normal to ask for 10% down to secure the purchase. If you are worried ask for his Dealer License and then call the agency in CA to make sure it is current. You can also ask your bank to see what type of account he is asking for the money to be wired to, personal/business?
I guess my question to you is why would you put money down on a boat if you have not seen it in person, and met at the place of business???????? I think you are asking for trouble, its not like boats are just flying off lots right now!!
BTW I do have my Boat Dealer License.