"If the marine industry really wanted to do something smart they would put a take off on the propeller
shaft in front of the stuffing box running a belt drive to a low rpm alternator. "
People have done that. The "marine industry" is concerned with making profits, not crafting boats, which is why it is an industry. And there's just no profit to be made in niche markets for tiny numbers, compared to industrial ones.
Before there was an internet people read books. Composed by authors, not sniping at tiny slices of subjects. I'm sure I've read about water generators, towed and shaft alike, in cruising books written in the 70's.
Putting an extra pulley and belt on the propshaft is easy. IF there's room in the engine space. IF there's room on the shaft. And IF you add a bearing to take the extra side loading off your stuffing box and cutless bearing.
Towing a genset is easy. If nothing eats it or fouls it and the seals hold. The latest iteration seems to be transom-mounted to avoid that. If they're also using new motor technology, rare earth magnets and the like, and getting enough efficiency...they're still expensive. And still need seals on a rotary shaft operating in salt water.
I suspect there's a reason why 30, 40 , 50 years down the line
, this "solution" still hasn't become mainstream.