In Ark Survival Evolved, I finally managed to tame a mosasaur and build a platform with default settings and no cheats. I want to build a small home on the mosasaur's platform. Now let's say I put in a door and have no openings inside the little hut. If I take the mosasaur underwater, will water go through the hut or will it act as a sort of submarine?
As has been mentioned in the comments and on Steam, a Mosasaur base will flood underwater.
Underwater bases, specifically, will use a new “Tek” tier of structures that are vacuum sealed. If the structure is completely enclosed, all doors are closed, and a unit per area called a “Venilator” is powered, then that volume will be free of water and you’ll have oxygen. If it’s not, you’ll be underwater and won’t have oxygen. You can construct air locks to provide a method of entering and exiting that doesn’t flood the inner compartments.