While your application is specific, the rules are the same. You have an energy need. You need to fill it. The energy need doesn't matter whether it's on a boat or an e-bike, or a golf cart. The fact that it's a boat has additional implications that non-boat owners may not grasp, and those usually whittle away at the best case, e.g., solar panels on sail boats suffer from shading horribly.
Consider that a single gallon of gas contains 33.7kWh of energy. That's about $8000 worth of LFP cells.
Good news is that burning gas is pretty darn INefficient, so you only get to use about 10kWh of that for propulsion or generation ignoring any other inefficiencies.
The implication is that for every gallon of gas of typical usage you replace with electric is going to need 10kWh of incoming energy supplied by batteries, solar, or shore power, etc.
This is a GROSS approximation and assumes you use the boat on electric exactly the same way you used it on gas. But it's pretty accurate for efficient EVs. They have about 40 miles of range per 10kWh.
So if on an outing, you use 2 gallons of gas, you're going to need about 20kWh of energy from some source... solar, battery, etc.
