I'm not a fan of Storm Giants being able to call lightning actually, (since I don't exactly know how a giant can get lightning to be directed without some sort of divine help) but this one makes more sense to me: Allow Storm giants, when on the water to be able to throw their mass into the water in an attempt to create a wave directed at a foe, group of foes.

The result would be that if hit by the wave, the affected person would take minor damage, lagged a short time (as per a bash), and thrown in a random direction per the exits of the room (with resulting possibilty of capsizing whatever boat they were on).

The result to the storm giant would be that he would be lagged a short bit (as per bash), would have his movement reduced a bit, and would be unable to attempt the maneuver again for roughly 3 hours (due to the extreme effort it took to .

The ability I believe, is good on a few points:
1) Utility in combat
2) Restricted by area of use
3) Fits in with the storm giant concept of being a water dwelling race.

Flame away.