Catalina Island is center point for Southern California diving activity. Tens of thousands of dives are made here every year by all experience levels and the heart of this diving Mecca has to be at the Isthmus. 

The Isthmus at Catalina Island is the narrowest point of the Island at only a quarter mile across. Straddling the strip of land is the tiny town of Two Harbors. On either side of the town are large coves, both offering varying degrees of protection from winds and ocean swells. Most easily accessible from the mainland is on the north side of the island with a large cove known as Isthmus Cove. It is in this area that you find some of the finest dive sites in all of California. 
While technically outside Isthmus Cove as you head into the area, the massive rock sentinel Ship Rock is there to greet you. The spire rises 66 feet over the waterline. Heavily incrusted by bird droppings, its white appearance from afar gives it an appearance like that of a sailing ship, sheets fully deployed, heading for the horizon. Closer inspection gives you a hint of the spectacular topography that lies below. To the southeast a finger reef projects just breaking the water. It has the look of a ship-gutting reef for those that approach too closely. This is indeed what happened in 1990 when the 90-foot wooden schooner Diosa Del Mar ran aground and broke up leaving a debris field on the steep sloping rock face. 
There are a number of areas to explore around this huge rock. On the south of the hook that destroyed the Diosa is an area known as the Fish Bowl. This is good place to start for the beginner to intermediate diver. All other sides of the monolith are vertical drops with ledges here and there at 15 to 30 feet leading to steep jumbled boulders slopes ending in white sand at 100 to 130 feet.
Blizzards of blacksmith fish swirl about in and out of the thick kelp. This is one of the best fish dives on all of Catalina with the usual Catalina favorites of garibaldi, halfmoons, señoritas, rock wrasse, perch and opaleyes, all in copious quantity. More sedentary reef fish here include rockfish and lingcod. In the cracks and crevices keep your eyes open for octopus, an occasional lobster, moray eel and horn and swell sharks. And on the deep bottom sand look for angel sharks.
Water clarity, with the exception of during plankton blooms, is excellent — up to 80 feet. Currents come and go and at times can be strong, but because there are several sides and angles to this rock it is easy to duck out of strong water flow. If you are skippering a private boat, use caution here as shallow reefs, winds, currents, and deep water can make anchoring difficult. Because it is outside the cove, weather here can sometimes be a problem.
As for diver skill level, a beginner can try here but only with a heightened awareness of depths and preferably with a more experienced diver as a buddy. 

Moving in closer to the island is the humpback rock known as Bird Rock. Second only to the Avalon Underwater Park on the east end of Catalina, this is quite possibly the most frequently visited dive site in all of the California. There seems to be a dive boat, private or charter, on this location 4-5 days a week. The constant parade of divers has however done little to reduce the charm, wonder and excitement of this dive site. Overheard on one recent charter boat trip was an experienced diver exclaiming with a smile on his face, “I have dived this site over a hundred times and I never tire of it!”
Most boats anchor on the northwest end of the rock over a shallow reef shelf varying in depth from 20 to 30 feet. The life filled kelp forest here is quite pleasant, not too thick and with openings here and there making it easy to move about. But this is not the main event. Head due north and the rock reef drops off vertically 30-35 feet. Along the wall are undercuts with hiding octopus, resting horn sharks and beautiful stands of gorgonian clinging to the rock face. At one point there is a huge gash in the rock cutting deep into the plateau. Lobster are common here although most are small. To the northwest at about 50 feet is a large swim through giving an underwater arch that is an excellent photo opportunity to frame a diver with fish. Back in the opposite direction you’ll come across a huge overhang at the base of the wall draped with gorgonian sea fans. 
Most of the diving depths here are above 80 feet. Water clarity is very good with 40 to 60 feet being common. Currents can come up across the boulders away from the wall, but as long as you are near the wall or over the plateau you should not have any problems.
Farther inside Isthmus Cove, just to the southwest of Bird Rock, is Isthmus Reef. While submerged, it is well marked by an anchored post with sign and light to warn boaters to steer clear (top of the reef peeks through the surface at low tide). It is a large reef with shallow plateaus, stair-step ledges, undercuts and vertical walls. Small caves hold abundant lobster, although most are small. Horn sharks, some quite large, inhabit the crevices. Across the top the kelp is thick so use your best kelp diving techniques.
This is a great spot for photography of the small critters of Catalina Island. Nudibranchs are plentiful as are small fish, especially juveniles of sheephead and garibaldi. While the water is a bit less clear here than Ship Rock and Bird Rock, the wide-angle kelp forest photo opportunities here are great. With the various angles of the reef it is easy to get in just the right position for a great kelp forest shot. Add in the schools of fish, bright colored gorgonian and a dive buddy and you have the making of an award-winning photo. 
There is generally little current here and enough shallow to moderately deep plateaus for this to be a good spot for the beginner. At the same time vertical drop-offs to as much as 100 feet of water makes this exciting for the more experienced diver.
In addition to these excellent dive there are many other spots to visit nearby — Eagle Reef, Lionhead, Blue Cavern and more. Schedule several trips to visit them all.
A dive log for the serious California diver would not be complete without a visit to the Isthmus and journaling on your adventures at these fantastic dive sites.