The real problem is the 'value" of the boat. Everything probably could be fixed for a grand or two at most by a professional. (Much less if you follow PB's solutions and do it yourself.)
Problem is that your contract says $12k...so you can't say...it's not worth that cause you already agreed it was. Your choices as I see them are as follows:
1. Walk away and eat the cost of the haul and survey. This is legitmate if you say you were not aware of the problems and don't want to go forward since the contract is "subject to survey"
2. Offer $10K and take on the fix of the problems youself. You can legitimately say that you will walk away rather than pay $12K for a boat with the problems found. If you love the boat and it is otherwise in great shape, this may be high...but not "bad".
The one option I would not take is letting him make the fixes and paying $12K. The pictures looked like it was in much better tha average condition but I guess they were taken a while ago. The good news is that even if you walk away...you've learned some stuff and there are LOTS more C27s on the market. Good luck!