Recently I have been trying to make a submarine that allows you to explore dangerous underwater places. Yet, I have no clue of how to make the submarine not have water in it. I am using terrain and I have not found a way to make it yet.
It depends on what you want to define as “water”, because obviously you can make it seem like your under water without any water at all - terrain is split into 4x4x4 cells and so although you can gain curvature you will never gain a perfect water balance.
An idea you might want to look into is a temporary submarine in the deep depths and then when your finally ready to explore a location it teleports the character to an appropriate area through a specified hatch. But to answer your question - it will be very difficult to use terrain water without the submarine having water in it.
You could however, set state of the player when they are in the water and then use color correction to remove the blueness of the water. This is done through Humanoid:SetStateEnabled() to which you can disable swimming in specific areas.