I just went back from La Paz.
I got 4 days diving at cantamar. The hotel is simple but ok, the food is not spectacular, and the bad think is the hotel is located 10 kms far from downtown if you want to have dinner in other place, but the price and the diving was very good.
We were lucky to dive with a whale shark the last day. simply spectacular.
We also dove with sea lions, turtles, snappers, angel fish, and other species, we saw a sea horse. Also we saw at surface lots of dolphins, mobula mantas, and pilot whales.
I think the best season to dive la paz is september and october.
Dive sites are about 40 minutes to 3 hours, there are lots of places, wrecks, reef, deep dives, and they plan it according to the dive group. They also divided the group in two when needed (advanced ones from new ones). Crew was very helpful.
Hope you can see the whaleshark too.