"It's hauled out in an expensive boatyard"
Welcome to boating.
You are learning the first lesson about boating - it is an expensive hobby. FWIW any boatyard is expensive, they split between just expensive, really expensive,a nd goqawful expensive...Getting an older boat free is like having parents who pay the 20% down on a mortgage for a first-time buyer's house - you may be in the house, but you will be paying every month - the only question is how much.
If you figure out the boat make and model, board members can give you estimates of base annual operating costs to consider, $100-$200 per foot is a ballpark, assuming you use your labor for all the maintenance work (typically 3-6 weekends a year, if no rain...). Of course the big question is whether the free vessel has been continually maintained and upgraded by the previous owners - if not your ownership could start to involve some real money.
Good luck, boating has a lot of rewards for the fans ( or are we fanatics?), but this free gift is likely to cost you a lot of money.