What kind of bug is this?

It lives along the water (brackish water). It lives on rocks that are lined along the water to control erosion. It is about 1″ long. Black on top, grey underside. Two grey antennas. It travels in large groups. Runs VERY FAST along the rocks. I have seen it for years. Never could get close. Quick moving. Any ideas? Not a spider. More like those pill bugs that roll up into a ball when you pick them up. Sub tropical climate.