How would I do this?

I'm looking to create an underwater cave system in my level. Unfortunately I've ran into serious trouble in that it appears the only water options I have to add water to my level are static water activators that can only be square in shape.

This is a problem because you can't "combine" water volume - if you try and cross one water volume with another it momentarily pops the player out of water right when he crosses into the new volume. This resets player breath, causes the screen to flash into a "not in water" state, and makes them fall to the floor. Am I missing something obvious?

The design of my dungion is focused on making the player head deeper and deeper into the cavern. I want them to be able to find pockets of air to breathe, until they finally reach the open area at the bottom of the cavern system. This really isn't possible though if the only way to get water to work indoors is by using these flat squares, because there is no way to vary the water level in a believable manner across the whole dungeon while having it be seamless.