I wondered the same thing about the rings and/or cylinder corrosion. I was thinking of taking my compression tester and one of those portable battery boosters to crank the engine over and test it. (Won't be starting it on the hard though.)
I'm told by the boat yard that the engine does crank over freely although they have not started it either. Carb is likely gummed up as you say...I'm assuming they're an updraft type on this older style flathead engine?
Do you think it's reasonable, or possible...on a boat that's been sitting like this one to include a sea trial as a condition in the offer to purchase? I'm not sure anybody would really want to set out in this without really checking it out completely first. Seems like that would put the onus on the seller to cover the costs of splashing it.