Unless the boat is small, old and or cheap, few buyers will put down a 10% deposit held by a private seller. BoatUS has an escrow service if no broker is involved in the sale
Also, bybthe time a buyer has spent hundreds or likely thousands on survey, haul out etc they won’t back out for minor issues. As to no deposit no sea trial, it depends on the buyers. You can often feel if someone is serious or just a tire kicker... what s an hour of your time and few gallons of fuel?
Couple of years ago I did a quick sea trial before contract and deposit on the boat i used to run. Buyers were from out of town, were supposed to fly out the next day and no time to get the paperwork and deposit. I had a good feeling about it, offered them the ride and that sealed the deal.