I dove in La Paz in October. They get about 5 inches of rain a year, and I got about 2 inches of it on my second day there
. But that was a "freak" thing according to the locals. I stayed at Costa Baja Resort and dove with Fun Baja, which operates out of the same marina where the Hotel is. People recommended both of these operators before I went, and that's why I chose them. They said they were the best in La Paz, and I was not disappointed. Would highly recommend both of them.
Fun Baja also just purchased a liveaboard boat that they cruise around this area. I've heard they have better prices than other liveaboards in southern Baja. But I haven't investigated. A liveaboard sounds like it might be a great (albeit more expensive) idea, considering that every day you go diving out of La Paz it takes about 1 - 1.5 hrs to get to the dive site (Cantamar boats would take even longer as they aren't as fast). And you'd have access to dive sites that people in La Paz wouldn't.
The diving was great, although Baja California is the only other place I've dove other than my local waters, so I'm not an expert in world class diving. Water was 82 degrees. Viz was 60 to 100 ft. The town of La Paz is not very interesting, though. In fact, Baja California is not a "rich" in culture as many other parts of Mexico, so you might be disappointed in this department if you were hoping for it. Other places of interest in southern Baja include Cabo Pulmo (a marine preserve), Cabo San Lucas, and Todos Santos. If you rent a car and you're adventurous, you might make a giant loop and visit all these places.