Assuming that you're getting rock that was once reef, similar to what Marco gets, I would say that a few weeks soaking in that small puddle off your back porch would make for some great live rock.
I'd take the precaution of soaking in a small volume of water for a few days, then testing that water for Phosphate, etc. before adding the rock to an established system, but I expect that it would be fine.
Bill will chime in here shortly, I'm sure. If there is a reason not to use it, he'll know.
BTW, you stole that view from a dream I have on a fairly regular basis........