No, it's not like buying a house. The "contract" in housebuying is a binding agreement to purchase, and (unless special contract to purchase has been drawn up which i bet it hasn't) the "contract" in buying a boat is simply the transaction itself- the swapping of money and boat.
So, the seller can agree to sell to you, and/or you can agree to buy. But you can change your mind right up until the transaction is completed and so can the seller. For each of you, if you have "shaken hands" to proceed, your only resort is legal action to reclaim your costs. But neither of you can enforce the sale to go through - the court sees the status quo - that the buyer has his money and the seller still has his boat - "no loss". There might only be a case for costs if the seller withdrew (survey fees and suchlike for you), and perhaps re-advertising fees for the seller, so hugely unlikely unless you also run off with his missus or otherwise make his blood boil...
Note that the transaction isn't complete until exchange of goods and property which means cleared funds and bill of sale. After that (even 5 minutes after that) the contract is complete.
This brings about the issue of the "deposit" which can be murky. Make it clear that it's a "refundable deposit" or a "non-refundable deposit". Best of course is a deposit that is so small that it doesn't matter - small enough for himn to re-advertise, say 500 quid should be enough for him to say "it's sold" to others. But he could withdraw and give you back the money even so.
At the sale, easiest thng for a buyer is to take a bank draft for loads of money minus a few hundred or so and check the engine hours. Or of course, take the whole lot in cash money, take a look, sound disappointed, walk away and at the last minute come back with a dropped offer - it' s very very hard to refuse - but don't expect a happy handover. And you're gambling the survey fee that the seller won't take umbrage and refuse your new offer and might (and could) even raise the asking price - because there's no contract.