I'm not a lawyer, but my understanding is that the contract isn't valid unless there is consideration (that is, both sides have something to lose).
A deposit means the buyer has lost the use of his money, and possibly the deposit itself, and the seller isn't able to offer the boat to anyone else. Thus there is an exchange going on.
If a contract is entirely one-sided, it isn't valid, so effectively if you don't offer a deposit, I believe the seller could drop you if a better offer came along and you would have no recourse.
As I said, I'm not a lawyer and everything I said could be wrong. This isn't legal advice.
Practically speaking, I bought my boat without any deposit, although it was for a smaller amount than you're considering.
You could probably offer a smaller deposit, in this market it would likely not be a problem, although the seller may question whether you can actually pony up the full price of the boat if you can't put down 10% (which any bank is likely going to require for the loan anyhow).