My advice: Ask the owner if it leaks. Then don't trust him and put it in the water anyway. Then you will be able to see what kind of leakage exists (if any). Why buy a boat that you have to patch when you can buy one that doesn't leak? I got burned really bad on a boat once....the guy was not honest and I learned my lesson.
Simply let your 'Yes' be 'Yes' and your 'No' be 'No'. Anything beyond this comes from the evil one. Matthew 5:37