As heading on left side of rockport up 127, pass Folly and when see andrews street, take left to end and go to right right when see wall.
Following is from Northshore frogman's web site:
Lanesville is a decent intermediate level dive. Depths range from 0 to over 60 feet, if you swim out far enough. The trouble is, distance here is measured in the number of lobster lines you cross. All season long you will probably find no less than three sets of parallel lines of pots, running along the shoreline, starting from just few hundred feet from shore. So, swimming out any great distance here can be a chore, as you must either pull your flag under with you, or keep surfacing to detangle. The problem with surfacing is that it makes you an easy target for the lobstermen who frequently tend their pots here. Nothing - it seems - can be said or done to prove to them that scuba divers aren't the reason their catches are declining - and the lobstermen at and around Lane's Cove are directly descended from Ned Beatty's friends in Deliverance. Trying to keep 150 feet from any lobster trap at Lanes Cove (as the signs claim you must) is like trying to stay 150 from a pan handler in Central Square. Basically, you would if you could but you can't.
After parking along the seawall, check the conditions. Head off to the right, as you face the wall, and climb up onto the rocks and look seaward. You want mid to high tide for your entrance, and seas that are calm enough that they won't throw you back onto the rocks you'll be sliding off to get in. If you go in on a receding tide, the exit can be quite strenuous! If conditions are acceptable, go back and gear up at your car, or carry only what you'll wear into the water, down onto the rocks. Leave your car locked and valuable out of site.
The exposure is northwest, and the coastline is fairly straight. Slab rock sloping to the water. Once you are in, swim out only a couple hundred feet and you will find yourself in 20 to 30 feet of water, with a very weedy, kelpy bottom. Lots of little rocks and holes with many lobsters around, although the vegetation is thick enough that it may be hard to locate and grab them without actually tipping them off. Other life you may encounter at Lane's cove include ocean pout, flounder, hake, skates, sculpins, and sea ravens.
Keep swimming out, paying attention to the ever-present longshore current. Try to head out and in to the current, which is usually blowing North. This way, when you return, also swimming a bit into the current, you won't be too far North of your exit point! On one dive at Lane's Cove a couple years ago, my buddy and I had to swim back from just outside Folly Cove! Once you've crossed three sets of lobster lines (approx. 300 yards) you should come to a sloping rock bottom that eventually goes well below 60 feet. There can be a thermocline here and temperatures can drop off 10 degrees or so. There is a fine silt on the slab rock here. Try to stay off the bottom and avoid kicking near it as well. The generally good visibility goes to hell and stays that way for quite some time if you stir up the inky silt. Here be the big ones, matey. Look in the cracks in these slabs for lobsters that used to be legal in size, before the 1999 upper limits were set.
All in all Lane's Cove can be a nice alternate dive site, when faced with unacceptable conditions or no parking at your favorite spot. You do need to lug your gear up and down some rocks, and the current here can not be ignored. But, if your in good condition, a good swimmer, and willing to put up with occasional Yahoo harassment, Lane's Cove is a decent spot to get wet.