Have you considered a solid oxide fuel cell to charge the batteries and increase the range? They can run on anything from ethanol, to LPG, to hydrogen.
Are there even any commercial products with these in them? I did a cursory search and the top 12 results were all research papers or research institutes. I found one company that produces the cells but nothing analogous to an internal combustion engine. A gasoline or diesel powered boat has a conventional engine block with intake and exhaust that's attached directly to the propeller. I don't see how any company could use a fuel cell if they can't buy it as a complete generator.
Bosch backed Ceres Power has a compact SOFC stack close to market in partnership with Nissan as a range extender for electric vehicles. The tech exists and cost more than within within reach for yachties. Bloom Energy are also players in the space. Honda has the FCT Clarity. Not sure why this tech isn’t being pushed more for marine uses; reduced price sensitivity and high volume manufacturing isn’t really a concern for a large segment of the market.