although the rock will be rinsed by fresh water, TAP water will work better.
the clorine in the tap water will help in killing hte micro organisms
also, at some point you will have to dry the rock and then rinse it out as good as possible.
things will grow in fresh water too, although they will probably be killed once put back into saltwater, but you dont want to import any 'nutrient', back into the tank
so, like i said, you will have to dry it out and rinse the snot out of it at some point
unless you do the 'cooking' method.. which is NOT real cooking, or the use of heat, but rather to keep the rocks in salt water in DARK, for months... and rinsing the rocks out every two weeks or so with newer salt water..
Lots of folks see/hear about cooking rock and they really think of boiling...
although i am sure it can be done, I would not rush to do that first