Ask the seller about the boat. Ask him when the oil was changed ("I forget" is bad); ask him when the air filter was cleaned/oiled or replaced ("uhh..." is bad).
Also, if he brags about how fast it is, or if he installed anything that would increase stock horsepower, then you can bet that it has taken at least some WOT runs, or worse. Be more suspicious if he is a 19 year-old with pink spiked hair and pants that hang at the knee.
If instead, he brags about how comfortable the ride is and how well it cruises (or sounds like he didn't just pin the throttle and flip the thing everytime he went out), you can presume it was not abused.