I can’t speak to ferries in particular, but it’s a feature of vessels of a certain size to have accommodations for crew. Even if it’s for short trips, you kind of have to have a space where crew can relax for a bit.

Most airplanes, even those doing shorter (4-6 hour) flights, have crew cabins. This, I know about. They’re typically located in the front half of the plane, accessible by hidden spiral staircase(s) in the front area, where the staff prepare food and collect drinks. It’s a short area up there, basically just pods to lay down in. The entire staff wouldn’t be standing at the ready on a plane. At any time, you can assume a bunch of them are up there, even if they’re just playing on their phones, reading, or catching a short nap, with an alarm set for the next meal/drink service.