Melan

Ah, yes, now I understand. You want to make it realistic from engineering point of view. That certainly makes it more difficult, especially if you need it to have an exterior, too. If it would have only an interior, it would be really easy.

My strategy, if I was doing it would be like this. Since submarines do not have windows, exterior is not needed. Just build an interior inside boxy sealing geometry. If you really want to show the submarin from the outside, you could build a separate area with an exterior-only version of the ship. Since the player needs to use an airlock to get it, you teleport the player to the interior version when the player accesses the ship.

Hmm I will give it a try, maybe it will make a big difference.