Maintenance is the key. When I sold my first SeaCraft, a '73 Seafari, it had 1700 hours on a '73 inline 6 cylinder 150 hp Mercury, and had 125-135 lbs of compression across the board still. I changed spark plugs, gear oil, and impeller every year or 100 hours. I checked ignition timing every year. (Never had to adjust it). I always ran the oil at 40:1, not 50:1, on the advice of Frank Brown. The boat had been in my family since new, and always got excellent mechanical maintenance.
My cousin ran the boat hard for two more years before re-powering with a Black Max 175. The inline 150 "Tower of Power" was still running great when he sold it with over 2000 hours on it. He just wanted more power. He didn't get it. Turned out the boat was faster and used less fuel with the inline 150.
Man, I
LOVE those inline 6 Mercury motors.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury