I think he's talking about something like an airboat
Airboat - Wikipedia, the free encyclopedia that's used in swampy waterways where a water prop would be fouled. I don't think they're really efficient, as they use a small aircraft engine to attain the sort of speeds you'd get from a much smaller outboard. And I really like the Wikipedia article's understated comment:
Quote:
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The engine and propeller are enclosed in a protective metal cage that prevents objects, e.g., tree limbs, branches, clothing, beverage containers, passengers, or wildlife, from coming in contact with the whirling propeller...
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PS: Just a note re small aircraft efficiency. With my Piper Cherokee 180, I could do about 18 mpg, not far off typical SUV mileage, but I was travelling at ~140 mph, and often cutting miles travelled by a third, or even half, 'cause I didn't have to go around curves.