It's hard to beat the State launch facility at Sandy Point State Park. I believe it's one of the largest and best maintained in the nation. There are several ramps in Havre de Grace, and you should NOT use the one at Jean Roberts Park - it is silted in badly and at low tide it would be impossible to get in or our. Better off using the one at Tydings Park. That water at low tide there is about 3 feet.
There's a country ramp in the Susquehanna River just upriver of Perryville on River Road. Great ramp but can be quite crowded on weekends. There's one at Owens Marina too, which is where I keep my boat - $7 a launch.
The Rocks in Cornfield Harbor and Cornfield Creek are not natural. Just outside the mouth of the creek is Jobs Rock, which is a series of concrete chunks that were dumped there many years ago to create oyster habitat. I believe the hard stuff inside the creek is there for the same reason.
About 6 miles up the Potomac on the north shore is where I usually anchor up inside Smith Creek. I also have access to a private marina there as well, which makes it nice in summer when you need electricity for the AC. The creek is fairly deep and well protected when the weather gets nasty.