If it’s anything like the electric ferries that cross the Öresund beween Helsingborg and Helsingør, they grab charge while they’re unloading and loading at each terminal:

Each trip consumes approximately 1,175 kWh, which is nearly the same amount a residential home consumes in a month. In each port is a tower with a robot arm that connects the charging cable automatically every time the ship comes to the dock. The system charges 10.5 kV, 600Amp and 10.5MW. The batteries have a total capacity of 4,160 kWh, which means that we always have a surplus of electricity if for some reason we cannot load during a stop or if the transit takes more time than usual.

In Helsingör the ferries charge for approx. 6 minutes and in Helsingborg the ferries charge for approx. 9 minutes. This is enough to suffice for the journey across the strait.[1]

Side note: you can also charge your car on board from the boat’s batteries.

[1] https://www.oresundslinjen.com/about-us/sustainability

10.5MW on demand is wild