If I place a water plane activator as part of a spell, the water appears as expected, but it doesn't act like water. There's no underwater visual effect and no one swims.

If I then leave the cell and come back, the water acts as expected.

Is there any way to make the water work like I want it to in real time? Or am I going to have to abandon this spell?

Gah, it actually DOES work like it should for a split second after appearing... then it goes back to being just a visual water layer. Video:
http://www.youtube.com/watch?v=e0otgWZKCUk

Solved this. The secret is to put the keyword "Movable" in the water activator you want to use, then translate it. The "swimmable" region moves with it.