I would say a $500.00 deposit if he is truly interested is a good place to start. But dont expect the deposit before you hit the water. He is out to test drive the boat, so make the test somewhat short. Let him see how it runs and so forth. At the end of the test let him know that you will need a 500 deposit to hold the boat for him and give him a time line to get you the rest of the money. Just my .02