What happens when you get a battery fire on a ferry?
Thousands and thousands of pounds of battery.
Every one of our nuclear submarines has thousands and thousands of pounds of batteries. More than a World war two diesel electric boat would have. They also carry diesel engines to charge those batteries with. Every nuke boat is also a diesel electric boat by design. Why? The reactor must be cooled at all times with pumps or it will melt down from decay heat. If the reactor scrams and you cannot go critical again for damage reasons you also need propulsion to the surface and then you snorkel home in diesel limp mode.
Hybrid electric boats make sense for ferries and tug boats as well. The most efficient way to run a diesel is not rapidly ramping up and down the power curve it’s running it at 75% of full load flat rated. Ferries have to start ,stop and maneuver in rapid succession this causes them have diesels sized for the max instantaneous load point not the max continuous load point they are no where near the same. Tugs have an even more peak load vs avg load curve.
The hybrid solution allows you to skip the gear boxes as everything is electric like a diesel electric train, plus you sized the battery pack for peak load amp draws and size the engines for the avg continuous load with a 25% upper cushion from 75% to 100% loads.
Overall you will save fuel as the engines spend more time at the best BSFC point even considering the mass of the batteries which have dropped drastically in weight and price with modern LFP cells vs lead acid or silver zinc of the older marine rated cell types.