I recently asked this question, in which one of the comments mentioned there might be problems with 3D combat.
My campaign will be nearly entirely underwater, in the deep sea where 3 dimensional movement is normal.
I'm looking for any potential problems that running 3D combat encounters will bring, and how to deal with them.
The game hasn't yet started, and I've never run 3D combat encounters before. My main concern is that all the flat AoE spells will become useless, since any target can move off the 2D plane that the spell is on and get out of the effect.
I'm not looking for RAW answers only. Any house-rules or homebrew spells and abilities would all be appreciated here.