You can take your chances finding legal on-street parking, or you can pay for parking at Halibut Point State Park a short ways down 127 towards Rockport. MA Magnolia Rocks is a rocky piece of coastline that slopes down to a depth of over 40'. The bottom is littered with boulders that provide shelter for quite a bit of marine life, including a fair number of lobster. This site provides a similar entry as Cathedral Rocks with much less of a hike to the shoreline. Once at the area, scout out the shoreline for good entry and exit points.
Exit can be a little more tricky.
The bottom is rocky with scattered boulders. There is a lot of life to be seen here, and lobsters are common. This spot does not seem to get as crowded as other Cape Ann spots due to the hike involved. This is an advanced dive spot, and one best left to those in good physical condition. Wear good hiking shoes and bring plenty of water.