Torque is not power. It is the twisting force that an engine can exert on its output shaft. I can produce 1000 ft lbs of torque with my back and arms, more than many recreational engines. I just need a 10' lever arm to do it. But I certainly can't produce that torque at thousands of rpm and drive a boat to 20 kts. That takes horsepower.
Horsepower is a measure of how much work an engine can do. Yes horsepower is definitely related to rpm in the formula you mentioned, but it is real.
It takes horsepower, not torque to move a boat at a given speed as the back and arm example shows. But that horspower can be produced in a number of ways. You can take an old Gardner diesel that makes its maximum horsepower at low rpm, say 1000 rpm and hook it up to a direct drive prop shaft and it will move a boat.
Or you can take a modern diesel that makes the same horsepower as the Gardner but at 3,000 rpm and hook it up to a direct drive prop shaft but through a 3:1 reduction gear and it will move the boat at the same speed. And its torque after the reduction gear will be exactly the same as the Gardner.
But the Gardner will last much longer running at 1,000 rpm that the modern engine running at 3,000 rpm. But it will weigh twice as much and be much bulkier.
So it is all a compromise between size, weight and longevity. Most recreational boaters choose the lighter, high performance diesel because they don't put enough hours on them for longevity to matter much. And it is the external stuff that always fails before the crankshaft bearings or rings wear out.