I think there are several possible ways this can be approched and all are fairly complicated.
The first possible solution is to move all characters around the actual ship, which is really complex and I wouldn’t do such an approach if it is not absolutely needed.
Another MUCH easier approach would be possible if you only have a specific amount of ships (like one or two). Than you could spawn specific streamed levels for your ships in a place of the actual map. In these ships you can walk around like normal and they aren’t moving at all.
In specific places (like windows and so on) you could place textures which show the view from different camera angles of the ship, which is flying through the open world.
This is imho a much much easier approach. Unfortunately you have to teleport your character into the streamed ship level when entering the ship itself.
I could imagine that this approach is much easier and also possible in multiplayer.
I cannot believe that it would be possible to move replicated actors in multiplayer around a flying replicated object without syncing issues and actors falling through other actors and so on (like in the first approach).