What really matters more is the upkeep. The engines are good for well beyond that.. provided they've been maintained.
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There is no simple relationship here, and it doesn't depend exclusively on the power of the engine. A lot also depends on the size and shape of the boat.
A lot
You can get at least 200k out of the engine itself. However, the front end equip such as tired rods, bearings, etc, are another story. All those parts start to break down around 60k-70k miles. If they haven't yet, trust me they will. The only other problem is the aluminum heads on the hemi, become a bit finicky, have to let engine warm up a lot before driving or else it will start to shake and then die out.
There are a lot of factors that go into determining this. What is the effective gear ratio for second gear? What is the maximum RPM of the engine? What is the rolling diameter of the tires? If you can provide the answers to those three questions, I can tell you the maximum speed.
40 miles x 5,280 feet per mile = a lot a lot divided by 60 minutes divided by 60 seconds = 58.67 feet per second