For such a restricted-use spell, 90% of NPCs are immune where you can use it, and it's just bland and ineffective in general. I can't really think of many situations that it's not just going to be more effective to use some other ability instead.

Suggestions/Ideas:

Most basic and bleh...just plain up damage to something useful?

Maybe give it some sorts of dual-damage attack instead of just drowning. Maybe make it so that the battering of these huge waves can cause some blunt damage so you can actually do something on these immune mobs?

But my favorite, would be make it have some sorts of utility. Comparing it to whirling simoon (the other popular terrain-based power), simoon does blinding, maybe bleeding, with some good damage. Maybe tsunamis could risk denting/shattering boats, or be able to move people directions, or broken bones, or I hate to suggest any form of lagging, but it'd make me actually consider using it, if it actually accomplished something, even if it's not effects that're greatly reliable.

Of course, I'd probably just end up being more bait for water shifters, but ah well.