How do I make a spaceship where I can walk around in a ship while said ship is moving freely?

after trying the portals with interior under the level approach it would appear that I will need to with the first possible solution. as many of the ships that will be created will mostly be open top. picture a sailboat as those are, for the most part, what I am working with. I would imagine a physics-based approach would be optimal. do you have any resources or ideas as to how that would work?