I used to hear - 100 hours equals 10,000 miles in a car - 500 hours = 50,000 miles - etc.
Obviously it depends on what kind of (boat) driving) you do, as well as "who" was driving.
Not much different than evaluating a used car. Just ask lots of questions.
Also - how was it maintained? Did they change the oil every XX hours etc? And... I would always find out (unless you're in the midwest) - has the boat ever been in salt water. If so, how did they take care of it each time? So to me, 310 hours would be OK if the boat was well-maintained.
Last - I'd find out why they're selling. Does it sound like a reasonable story? Or has the boat been a problem? Get records where you can. Call the dealership that has worked on the boat. etc.