La Paz is on the Sea of Cortez, which Cousteau called the world's aquarium and which has more or less earned it. Almost everything here happens on the water, and most of it is seasonal.
Whale sharks feed in the shallow bay roughly October to April. The zone is protected: a fixed number of permitted boats, a time limit in the water, no touching and no fins near the animal. You snorkel, you do not dive.
Mobula rays gather in spring — thousands of them, sometimes leaping clear of the surface. Orcas work the Cerralvo channel. Grey and humpback whales pass in winter. The sea lion colony at Los Islotes, off the north end of Espíritu Santo, is there all year and the young ones will come and look at you.
Espíritu Santo itself is a UNESCO biosphere reserve: red rock, white beaches, no development, reachable only by boat. Day trips and overnight kayak-and-camp expeditions both run from here.
Divers get the Salvatierra wreck, the seamounts and the same sea lions. Balandra, on the road out to Pichilingue, is a shallow turquoise bay that is regularly called Mexico's most beautiful beach and has a daily visitor cap to prove it.