With all due respect to previous posters, I would def recommend against an organized trip in La Paz. There really is no need. We were just there in mid-Oct, and rented a 3BR/3BA private villa with infinity pool on the hill overlooking the Bay of La Paz for $200/night. This was the place: http://www.vrbo.com/249925
It was just me and a dive buddy, but for that price, why not. (There are a number of other similarly priced villas on vrbo.com if that one is booked.) I highly recommend diving with Cortez Club over Club Cantamar. They were fine with my DSLR setup, and had a well-run operation. They took at most 6 divers, and on two of our three days, it was just my buddy and I, the dive guide, and boat driver. They even did the 1.5 hour trip out to El Bajo for just the two of us (rather than the typical "we need at least x number of people before the long trip").
have a great time, in 3 days there, we saw whale sharks, a school of 30+ hammerheads, and had a number of great dives with sea lions. I unfortunately had a camera malfunction during our dive with the hammerheads, but our guide caught some video of them with her P&S.
video here: http://www.facebook.com/v/641251719281