Go HERE, check the options, click continue, then go to the second page after that and you'll have an idea of how a lender would value the boat.
Then contact a broker and get soldboats data on it so you know what buyers are paying for it.
If you contact the listing broker, ask how old the pictures are and if they accurately portray the boat as it is today. Ask about storm damage or if it has ever been submerged. If it's the one in Puerto Vallarta, ask why the owner is anxious to sell. Ask every question you can think of.