If you were to use a large amount of water, the ground would become loose.
Well, some water should be fine, I'm sure. There's earth magic, as well.
Even though I say that, if while using earth magic you unknowingly start using the earth from base supports inside the cave, then it could be a problem.
If you take out the support for the interior of the cave, it's possible it might lead to a cave-in.
Then, I'm sure ice is safest.
The safe choice is to go with what has been suggested.