In short, the diving was awesome. You do need to wear some sort of suit for protection against the microscopic stinging jellies; a dive skin is adequate. A couple times, especially at the El Bajito site, we got hit by some very cold currents/thermoclines, when a 3mm wetsuit would've been nice, but overall the dive skin is fine. The water temperature was, overall, in the mid- to high 70s.
Most of the sites are at least an hour or two away by boat. Club Cantamar, located south of La Paz, is considerably closer to these sites than other resorts/boats located out of La Paz itself, but if you're prone to seasickness, take appropriate measures. We were lucky in that the sea was pretty calm the whole time.
The boat left around 8am, and we didn't get back to the hotel till 5 or 6 pm. Lunch was served on the boat, usually sandwiches or soft tacos, chips, cookies, real sugar sodas, and fruit, such as watermelon. It is basically all you can eat. Snacks between dives were fruit, sodas, chips, and cookies. Plan on two morning dives at one site, lunch, and then a shallow dive at another site.
Dive sites included El Bajo (where we saw the juvenile whale shark on our first surface interval!!!!), El Bajito, Los Isoltes, the wreck of the Fang Ming, and Swannee Reef (a gorgeous, shallow dive with walls of fish and a wonderful "bed" of garden eels - what better way to spend time than to lay on the sandy bottom at 30 feet, gently breathe, and watch the garden eels poke their bodies out from the sand!
Other critters besides the whale shark included all four types of morays (green, jewel, zebra, and one other I can't remember the name of), sea lions, nudibranchs, an awesome school of barracuda, a huge school of mobula rays (similar to mantas but smaller), various puffer fish, and more.
The dive crew was very helpful in getting you on/off the boat (both at the dock and at the dive sites) and were happy to either guide you on a dive or, if you had the experience there, to plan and do your own dive.
Club Cantamar has lockers right by the dock for your dive gear. They supply tanks and weights; you can rent the rest of your gear there if you didn't bring your own. Rinse tanks, sinks, and hoses are adjacent to the lockers so that you can take care of your gear.