If you're staying close to Long Beach, I'd say Catalina is your best choice. There are 3 boats to choose from, one of 'em will have an opening. Barring that, you can still take the Catalina express out and dive at the underwater park; if you don't care about the cost, the boat dives are better than the park, but the park is pretty easy to do. If you're further north (i.e. North of the 101/405 interchange), consider going with a boat to the Channel Islands out of Ventura (The Peace has been my favorite SoCal boat to date). There are 3 boats out of Ventura, so, again, you're almost certain to be able to get on one. Or, if you're staying in the south, go for wreck alley. The thing is, you can wind up driving a looooong way to get to the boats, depending on where you're staying. If you're on vacation (even if its just one day), you don't want to waste it in So. Cal. traffic. You will have to waste your chance to sleep in, the boats all leave from 7 to 8 AM. I've been to Minneapolis, if you've never been here, you'll find that Southern California is a lot more spread out - nothing is very close to anything else by mid-west standards, although you do get the advantage of not having to screw around with ice on the roads. Given your time frame, expect the water to be around the mid-50s at depth pretty much anywhere you dive. It could be warmer, just don't expect it and you'll be okay. That'll be quite balmy for you...
BTW, while I'm not opposed to diving with you next March, its a bit early for me to plan on it. You might want to wait until the date's just a bit closer before looking for a buddy. You'll definitely get responses if you post that you're looking for a buddy next March.