Here's the deal regarding the boat running OK with low compression.....say the compression was low, but just above the lowest limit where poor performance would be easily noticeable....you could still accelerate, and still run at high speeds, but it might last a week, a year, or....the better news is, if it was way below necessary compression in even one cylinder, that cylinder would most likely be "dropping", and you'd notice it on acceleration and at top end......she'd sputter or hesitate on acceleration, for example....way below minimum necessary spec on two or more, and it would be most likely be hard-starting, and you'd have real ragged acceleration and top end. I'd still be concerned about being told the compression "is 105"....that answer doesn't make sense.
If you're getting a good deal on the boat, and the boat itself is sound, and the motor does quit, you might be able to repair or repower and still be around the resale value....just keep in mind an old boat isn't like an old car....even if you can do the work yourself, it's almost impossible to completely do over a boat, motor, and accessories and stay below or even at the actual value of the boat, unless you're getting it for next to nothing, and even then, maybe not.....the woods are full of boats somebody got fro free, then turned into planters when they found out how much coin was going to have to go into their bargain to make it seaworthy.....on the other hand, a little patience, and you might find a good deal on a boat with no problems and a warranty of some kind, and not have to worry about the extra expense and lost time due to repairs. Up here, a rebuild job is going to cost you the better part if not all of the season.
If your in love, and this is the only one for you, consider this.....Madhouse is right on about the dealer probably wanting to sell the owner a new boat, and the sale may be conditional on selling the old for a certain minimum price....the dealer may be willing to gamble on no problems showing up for say 7 days, and may give you that much warranty. Let him think you're dumb to settle for a 7 day warranty, who cares? If he's dumb enough to give it to you, head right down the road and get it gone over completely. Let us know how you make out....I saw your post in the other thread, and I know you're rightfully excited and proud, and I hope it all works out OK for you.