Technically speaking, San Pedro and Long Beach are both the Port of Los Angeles, but the spots the cruise ships leave from are quite a bit apart. In San Pedro, they leave from right next to the Vincent Thomas Bridge. If you go over the bridge, travel along Terminal Island for a bit and head over to Long Beach, some of the other cruise ships leave from next to the former Spruce Goose dome, right beside the Queen Mary. Can't remember which cruise line that is, though. Carnival, maybe? But, since jilljill said they show the Magic leaving from San Pedro, it sounds like everybody's on the right track. San Pedro and Long Beach are probably something like 5 miles from each other (oh, I'm so bad at judging distance!). It's something like that, anyway. Not far, but you couldn't walk it (at least, I wouldn't want to walk over the Vincent Thomas Bridge, even if it's possible).
jilljill - I don't think I've seen any car rental places at the port. You'd have to rent from LAX and keep the car at the port while you're gone. That would be a waste of money. Unless somebody like Enterprise has one of those neighborhood rental places in one of the local hotels. It's not really a big volume area, in spite of the cruise ships. You'd be better off with Super Shuttle or something of the sort.