I'm trying to create a massive vertical underwater cave system but after reading all the wiki and Tamriel Rebuilt tutorials I can't find any mention of adding water to an interior. It's not an object since I can't click it in other cells to find the id and it's not under the landscape editor settings so throw me a bone, where is the function to add a water table.

Assuming that was a simple question to answer for this vertical cave system I understandably want a feeling of descent but every large cave room only has one entrance and I expect players to quickly realise if I just flip a bunch of tunnels on their X axis. I tried flipping the floor sections of the cave statics list to act as walls but every one of them includes a roof segment so I'd just be creating extremely narrow pits. Can I get away with duplicating hundreds of 2.0 scale boulders to act as walls or am I better off just flooding a 2.0 scale copy of Akulakhan's chamber once I find out how to add water?

World > Interior Cell, select your cell there and tick "Has Water".

That was fast. Thanks.

While you're here do you have any recommendations on creating open vertical caves?

Look at Urshilaku, Laterus Burial and Moril Manor Courtyard for inspiarion. Both have large vertical area. Should give you some idea.