Take the case of a boat that is perfectly propped to hit its rated rpm at wot. Then drop down several hundred rpm to a comfortable cruise speed, say 2,000 rpm and 15 kts. It will take x hp to move the boat, say 200 hp for argument's sake. It will also take y ft lbs of torque as well, say 150 ft lbs (yes I could calculate the exact number but I am lazy, so bear with me).
Then take the same boat and engine and take out 3" of pitch. To get to the same cruise speed as before is going to require reving higher, say 2,300 rpm. The hp will be approximately the same- 200 hp. The only difference will be prop efficiency and it might be better or might be worse. The torque will be proportionally lower, about 130 ft lbs.
So which is easier on the engine: 2,000 rpm and 150 ft lbs of torque or 2,300 rpm and 130 ft lbs both at the same hp. The latter. Rpms at these levels doesn't hurt an engine much. Torque does because it comes with more heat and increased forces on internal parts.
Of course propping is a compromise and at extremes: 4,000 rpm and 75 ft lbs probably won't be so good on the engine either.