Here in Long Beach an owner has to sign a bunch of papers to get the boat hauled. The owner agrees that s/he is responsible for all accrued costs. There must be insurance on the boat. After the yard work, the boat must be launched, or be ready (i.e., all paid up) to launch, by about 9:00 am of the day it is so scheduled or the owner may be charged a lay day. To receive a "launch ticket" for that day, the charges must be settled by cash, check, or cashiers check. A potential buyer would have no role in any of that.
I would not expose myself to those charges to facilitate a potential buyer's "interest survey" w/o CASH in advance; minimally the full cost of the hauling/launching of the boat, etc., and one refundable lay-day amount in case the surveyor is delayed or in some way is not able to complete the survey in the 24-hr grace period for lay days. That is, out in the morning of day one--back in the water the morning of day two, or else risk a lay day charge. For a 40' boat, I'd be holding about $600-$700 of the potential buyer's dough. None of it could be applied to the purchase of the boat. But, if the boat was put back in time, he'd get about $200 of it back, It would be more $$ for a 45' boat.
Also, haul-outs may require a few days to a week's notice. Those guys are busy. A buyer maybe wants to buy a boat on Thursday and wishes an out-of-water-survey. It may be the following Wednesday or Friday-week b/f the boat can be hauled, and the owner/broker has to get the boat to the yard, etc. In the meantime--is the boat still up for sale? If the potential buyer does not lay down some cash, then it is. If he does but then does not like the survey results, does he get it back? What about the week+ when the owner/broker could not accept an offer if another came along b/c of the earnest money "deposit"?
I don't think a buyer "looking for a deal" is going to hand over some amount of money to recompense an owner/broker's time lost dicking around with the guy who now does not want the boat, etc. It's not a matter of trust, IMHO. It's business.