Lithium ion batteries produce it's own oxygen sadly, it'll continue to burn even when submerged under water. 🤡🌎

If you really wanted an electric ferry, the right way to do it would be to make it like a cable car ferry, especially over such short distance were there isn't much cross traffic from bigger boats. The cable will then be pulled from the shore by any type of engine really, heck if the engine breaks down it could be pulled by hand too.