It depends on the specific ferry and its security measures. Many ferries have dedicated bike storage areas or bike racks, and generally, these are quite safe, especially if the ferry company has a good reputation. However, it’s always a good idea to check if the ferry provides any secure locking mechanisms for bikes.
If you’re travelling on a smaller or less supervised ferry, or you’re uncertain about the security on board, it might be worth taking a lightweight lock just for peace of mind. If you know the ferry is reliable and has proper storage, you might be able to skip the lock and save on weight.
You could also consider asking the ferry company about bike security to make an informed decision! Similarly, don’t forget to insure your bike before travelling. I think temporary bike insurance coverage will be cheap. You can probably use this site to compare your bike or vehicle quotes. I personally don’t transfer my vehicles through ferries without insurance.