There are two major dive boats currently operating out of La Paz (to the best of my knowledge): the Don José and the Rio Rita. The Marisla, one of the first live-aboard sport diving operations in the Sea of Cortez (if not anywhere), was damaged beyond reasonable repair in the summer of 2001, and will no longer be available for dive trips.
There are also some smaller operations running day trips on pangas to the most popular dive sites just north of La Paz.
Caution. If safety is one of your concerns as a diver, as it should be, then I would recommend that you make sure the operator of your panga is knowledgeable about diving techniques and problems. I've seen a few dangerous situations develop with divers going out in a panga with a single operator who, it would appear, knew nothing beyond being able to operate the outboard motor.
The prime diving season in this area is July-November. Generally, at this time the water is clearest (there may be exceptions depending on the amount of recent rainfall), and also the water is warmest. Nothing like diving in 85 degree water!
I am most familiar with the Marisla since the owners are old friends as well as landlords (I permanently keep a travel trailer in their RV park). I am not much of a diver due to an ear that does not clear well, but I enjoy snorkeling and, most of all, boating. I have made several trips on the Marisla as an informal guest. Although the Marisla is no longer running, my experiences in diving the area near La Paz have all been wonderful ones.
A new entry to the La Paz dive-boat business is Scu-Baja. They currently operate two high-speed boats on day trips to the major dive areas just north of La Paz. These are open, shade-covered boats with fresh-water showers. Each boat can take up to 18 divers. They apparently also offer overnight trips with camping on Isla Espiritu Santo.
Another new entry to the scuba diving business in La Paz is the Cortez Club. They operate from a new dock at the La Concha Hotel north of the city of La Paz. Besides day boats, they have a cabin cruiser available for groups of up to six divers. There is much promotional information available on the website linked above.
The Rio Rita makes day trips out of La
Paz to the closest islands - I believe a one-week trip will have the
group staying in a hotel (e.g., the Los Arcos) and then taking the boat
back and forth to the islands each day. The boat is operated by
The Don José offers live-aboard trips on a reservations
only basis (i.e., making sure they have a "full" boat). On a
one-week trip the first and last days and nights are spent in a
hotel. The boat is medium large (larger than the Rio Rita and
smaller than the Marisla) and is fairly modern. This boat is
also operated by