take the ferry to catalina
I'll second the ferry to Catalina. And here's why:
If you're flying through LAX, the Catalina Express out of Long Beach is in the vicinity. Check out a map and see where LAX is in relation to Long Beach. If you only have a couple days in California, you'll likely fly into LAX and check in to a hotel nearby, so Long Beach will be close to where you're staying (heck, you could stay in LB, since LAX is close.)
There are plenty of dive boats that go out of LB, but they tend to do day trips to Catalina. They leave early in the morning and you're on the boat for roughly 10 hours... half of that in the crossing to Catalina and back. After flying in one day, and flying out a few days after, you probably don't want to spend a 10 hour day on a dive boat in between. And you probably don't want to subject your new bride to this either.
In contrast, the Catalina ferry is bigger, faster and more comfortable than a dive boat. It takes a bit over an hour to get to the island (and an hour to get back.) You have a variety of departure and return times to choose from.
The Catalina dive park is a short walk (about 0.8 miles) from where the ferry docks. The dive park is an easy entry and exit (a concrete staircase that ends in the water), and a local dive shop has tanks, weights and gear available right at the park (literally about 50 feet away from where you enter the water.) Although you can bring your own gear if you like, plenty of divers from the mainland bring their own gear on the ferry.
Visibility in the dive park is nearly always much better than shore diving from the mainland, and the dive park hosts a variety of life from tiny to the 400 lb giant black sea bass. Plenty to explore and appreciate for two or three dives.
When we spend a day at the dive park, we typically have a late lunch or early dinner at a local restaurant (plenty of them on that 0.8 mile walk between the dive park and the ferry dock) and then take a nice relaxing ferry ride back to Long Beach. We've never had a bad day doing this.
For a quick, one day of diving it's definitely the easiest and most rewarding way to go... especially if you're in the vicinity of LAX. San Diego is nice, but it's a bit of a drive from where you'll likely be staying and has more potential to have little things go sideways that could cause you to start questioning your decision making... definitely not something you want on a honeymoon.
