[P-hack]

I would say a yanmar on biodiesel is your best option, weight (and reduced range) is a much bigger penalty for things that must "float" on a fluid (boats and planes) vs things that roll on wheels. And there isn't much regen opportunity but a lot of time spent at a constant speed (where the losses of mechanical to electrical to mechanical will be amplified).

Biodiesel will get you your green merit badge as it is renewable. Electric/hybrid planes/boats don't make a lot of sense to me in the bigger picture.

an electric trolling motor on a kayak (with an RC airplane prop mod) makes sense just because it is ridiculously inexpensive to set up.

Note, the coast guard has some regulations about internal combustion engines to consider also.