Check local regulations. Some places, the rock cannot touch the ground. If it does, the ocean owns in.
@Eagle_Steve does this and calls them dock rocks.
@Torps
The regulations vary from state to state, but most states do not allow you to take live rock. But, if smaller pieces, think seashell size, are taken with some shells, the FWC is not going to write you a ticket. Take a 5-10lb rock, they will.
In Florida, you can use mined rock or any other thing to aid a crab trap in not getting moved by the tides, or keeping it stable when not on the bottom and being used for swimming crabs. This is a loop hole to allow someone to take previously dead rocks, throw them in the ocean, leave them there and let them become "live rocks". The trick to this is that they can never touch the bottom outside of the trap. I have a dock on the Indian River Lagoon that I use for my boat and a few crab traps. The traps are tagged per regulation, affixed to the dock with a stainless chain, and stay there year round. When down there, I bait them up and do catch blue craps and stone crabs with them. When not there, they are unbaited, trap door is open, and they just soak. I currently have about 25lbs of rock in each one soaking as well. The rocks are not like you would find from TBLR or KP in terms of the amount of life. They do have coralline, sponges, feather dusters, and macro on them. But not corals, larger starfish, etc. Most of the time they end up with smaller crabs, pods (lots of them), juvenile fish (all released back at the dock) and the occasional urchin. I have also never had an issue with any disease from them. The "open ocean" is a lot of water with a lot of tiny mouths. Ich for example is the size of what a lot of corals can consume. So all that water, all those mouths, ich is not a problem. It only becomes a problem in a glass box with not as much water and more of a chance for the swimming stage to attach to a fish. Not saying you cannot get it, but the chances are extremely low IMO.
I will say, that in using older rock and swapping out rocks from time to time with dock rocks, my tanks stay more stable, do not have as many issues if something does go out of whack, all of the fish pairs spawn, corals that are on deaths door at the LFS from a leaky bag/just a half dead coral sent almost always recover, and overall the tank just does well. In addition, I do not have algae issues, even when I set up a new tank with them. There are just too many competitors for the algae to get a foot hold and take off. Not saying you could not end up with massive algae, but proper husbandry and the rock gives you a nice head start.
