I don't know if your boat is an inboard or not, however, I sure wouldn't like to be around any gasoline that can enter any enclosed compartment.
Diesel fuel won't explode at atmospheric pressure. Diesel engines can be had in 'inline configurations.' A 4BT Cummins or 6BT should power your boat nicely. Early Dodge (non-computer) engines are plentiful and cheap and you could build a great intercooler with your free supply of cool water. Detroits are also good-a 4-53T-would do nicely, and there is lots of marine gear for them.
