Consider also that the 5.9 (and now 6.7) Cummins B motor is used extensively in pickup and medium duty trucks. A pickup driving at 70mph on a highway trip unladen typically gets 20mpg (3.5gph) and runs around 1500rpm. That's about 65hp. These engines are rated at around 300hp-350hp so highway cruising is nowhere near 80%. Even lower load putt-putting around town. Known to run easily to 500k miles, which at an average speed of 40mph, that's 12,500hrs.
Not too far from what trawler duty cycle looks like to the engine. Engine does not know what it is pushing, it just knows it is supposed to turn fuel into spin (and heat and noise!!). Truck, boat, gennie, whatever.
Lots of things to worry about with these boats. Engine duty cycle is very low on the list. It will do what you ask it to do. If duty cycle is reasonable, it will not harm the engine.