The only difference between boat engine and car engines is that the runtime is measured in hours instead of miles. How long have these engines been running since you put the new pistons in? How big is the boat? What kind of prop are you running? You may be overloading the motors because you have too aggressive of a prop. Those motors should be safe to cruise at 3500 if you did a good balance job on them, so you could use a shallower pitch on your prop and still get the same speed. It just seems odd that both motors would be having the same problem.