All the other domain abilities are 30' so we'll go with that.
That ability is super ambiguous though. Does it summon a wave from nowhere and if so, what happens to the water and how much is summoned? If not, does it only work in water or within range of standing water?
Since the icicle ability that it replaces is a conjuration-type effect that does damage, I'll go with the more generous interpretation which is that this is also conjuration and you create an instantaneous water wave that can be used for a manoeuvre, whereupon the water disappears.