*not an rpg player*
Remember the fomor in Even Hand had no trouble with flowing water.
the fire alarm fizzled - but not before setting off the automatic sprinklers. Water began pouring down from spigots in the ceiling.
Mag looked up at the water and then down at me, and his too-wide smile widened even more. "Really?" he asked. "Water? Did you actually think water would be a barrier to the magic of a fomor lord?"
Running water was highly detrimental to mortal magic, or so Gard informed me, weather it was naturally occuring or not. The important element was quantity. Enough water would ground magic just as it could conduct electricty and short-circuit electronics. Evidently mag played by different rules.
Edit to add: Earlier in the story Gard tells Marcone that "The fomor pratice entropy magic." [finish edit]
Something tells me that Water Mages approach magic from a substantially different... paradigm such that water may have different qualities. Look at the comments that Billy makes about how Harry and Bob's PoV on shape shifting is spell centric but an actual shape shifter is likely to have a completely different paradigm that probably has it's own set of rules (somewhere around page 83 of your story). (well I'm adding a lot of interpretation there)
Generaly water magic is a force of entropy... inhibiting magic, tearing appart projectiles in Carlos' shield... but I'm guessing it can also be used as a basis of creation like LTW's healing magic.
Interestingly enough, Fire is also a purifying force that can disrupt magic too... if one were to theorize, there might be some dicotomy here that I am not sure where to go with. Maybe disrupt a water mage's tracking spell with a burnin ring of fire?