if your ok with it possibly not working, i say go for it!
-use very long spindle pivot tubes so they extend from a beam welded to the bottom of the boat, all the way to another beam welded a few inches over the edge of the boat. put pitman arms on the tube so they are above the edge of the boat.
- stainless chain to aluminum axle, sprocket, rims. (i think you can also buy quality plastic rims for go karts.
- make sure tires are fine in the water and don't float too much.
- put a jack shaft on the edge of the rear of the boat, then you can put the engine inside the boat instead of hanging off the edge.
-either get water proof bearings, or make water tight boxes to surround the bearings that bolt to the boat with a gasket between them and the boat.
-make sure the motor doesn't hit any components, and that the new wheels and such aren't to likely to get hit, or cause major issues with the boat.
- find brakes that that can survive under water. (they only have to work on land, but they can't be damaged by going under water)
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or
make a floating boat trailer, tow the trailer with the boat, then put a low geared go kart on the trailer,
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or, like TS said, get a cart, install pontoon's, get ATV sand paddle tires.