Depends on what you like to see.
The Coronado Islands are quite far (20 miles?) from where a lot of the SD boat dive ops are located. It's generally known as a good place for novices and basic OW students, since there are several fairly shallow dive sites. Underwater you'll see a lot of brittle stars there. There are some seals and sea lions that make their homes nearby. If you're lucky, they'll come out to play with you. Honestly, though, the aquatic life at other, nearer dive sites is much more abundant and interesting than what you'll see at Los Coronados.
If you like wrecks, the Yukon and RubyE are relatively close by in Wreck Alley.
If you like kelp, the Point Loma Kelp beds aren't too far either.
Shore diving can be done easily at La Jolla Shores and La Jolla Cove.
On the way out to the Coronado Islands is the Hogan wreck. That's a fun one but more advanced than Los Coronados.
One of my favorite dive sites is Scripps Canyon. It's an impressive wall dive, but access can be limited from shore. Not too many boat dive ops schedule it because it's an advanced dive and there are easier dives to satisfy a broad spectrum of experience levels nearby in the kelp.
For an out-of-towner looking to do 1/2 a day of diving, I'd recommend diving: (1) Point Loma Kelp beds or (2) Wreck Alley.
The topography in the kelp beds can be rather interesting (Train Wheels, Ancient Sea Cliffs, 7 Fathoms), the marine life is abundant (nudibranchs, rays, fish, sometimes sharks), and it's so close. If you've never dived in kelp, you're in for a treat.
For SD diving this time of year, most people wear a 7mm wetsuit or drysuit with appropriate undergarments. A thick hood, booties, and 3mm or thicker gloves are a good idea. Expect water temps to be approx. 52°F at depth and 60°F near the surface.
SD boat dive ops to check out include: the Lois Ann, Waterhorse Dive Charters (Humboldt), and Marissa Dive Charters.
If you need to rent gear, Sport Chalet has multiple locations in the area.
There's also Ocean Enterprises in Clairemont Mesa.
Call ahead to do rental price comparisons. If you rent gear at Sport Chalet on Friday evening, you can return it by closing on Sunday evening and only pay for "one rental day."