We recently purchased our boat, which had already been put on the hard for winter and was blocked in by other boats--basically no way to do a sea trial. We negotiated an escrow which ended up being a little more than 10%. We still did a survey (of course), and we ran the engine up to operating temperature, for a solid 20 - 30 minutes. The seller also provided complete maintenance records going back over a decade, etc on the boat.
The sea trial will happen when we splash this spring, but we do already own the boat. The surveyor will attend the sea trial, as will the listing broker, and determine if there are any items that should be covered under escrow. Basically we'll be looking for anything that we couldn't check in survey--engine and rig under load, some electronics (like radar on the mast, depth sounder, etc.).
Basically, I think this is a question of comfort with the deal. Our seller and listing broker have been reasonable to deal with, and we have been the same in return. Our boat is in the northeast, where this escrow thing is very common.
Now, all that said, if I was buying NOW (in March), I'd probably make the deal contingent on survey AND sea trial, because it's close enough to Spring to wait for that. When we bought our boat, it was December, so it was harder to do that without risking losing the boat.
1984 Sabre 34 Mk I