> Question is can they get enough power to be useful. Marine diesel engines are very fuel efficient, fuel oil is very energy dense, and they need to run for a long time.

Washington State Ferries refit one of their largest ferries (190-car capacity) as a hybrid recently. They replaced two diesel engines with batteries, leaving the other two engines in place. This provides for significant fuel savings. When shore charging is implemented, fuel use will be reduced even more.

I imagine there are also uses where batteries fully replace engines. I've got a tiny boat for casual fishing, you tend not to use the engine that much, and electric propulsion is likely quieter and can probably modulate to slower speeds which could replace a separate electric trolling motor.