Boat make has nothing to do with your question. Need make and model info on the engine. Then post it in the appropriate engine forum located a bit under this forum.
most likely the boat sat long enough that the motor siezed up due to rust in the cylinders. could have been sitting for a really really long time, could have mice in the one cylinder, could have not been properly winterized and the block cracked.
any of the above can be fixed. some will take replacing everything, some will simply be a re-bore and hone. I wouldnt buy the boat
nugget of wisdom - never buy a non-running boat that you cant take for a ride.