I've done thought this through, long ago. Use gears whenever possible, as belts lose ~20% of the power (slippage). It's easy enough to make a special rim which you put on the rig when powering the marine drive, with a nice toothed gear welded to it.
Gears do lose power, but a lot less.
Also, in the Gulf, you'd face real weather, so consider robust V hull (the Steem Dreem has a nice hull), and making the van of fiberglass or stainless. At the least, superlative maintenance and ongoing rust proofing will be requisite.
Ordinary steel and iron last but a year or two before starting to flake away, without constant greasing and painting, in contact with salt water. Zinc. Think Zinc, as sacrificial zinc plates preserve steel by attracting the ions that cause rust.