For anything beyond a high-output alternator, because of space in the boat and limitations of the crankshaft bearings making it difficult to run really large loads off the auxilary output, it will ultimately be much easier to use either a cheap
inverter or a Honda portable generator.
I chose the
inverter option (see harborfreight.com; mine is 3000 watts and cost $160); it took two hours to install counting me crimping my own power cable fittings. It runs a heat stripper and a small shop-vac simultaneously. I have the stock 35-amp alternator on my A4 and of course it is a huge drain on the batteries, but it works.
Harborfreight also has a 10kW generator head waiting for a 20+ HP engine to attach to it -- if you look at it you'll realize it will never fit in your boat.
The Honda generators are powerful and quiet enough to be considered engineering marvels but storage and maintenance complexities, plus carbon monoxide fumes, turned me off.