We spent a land-based week in Los Barriles, La Paz and Cabo Pulmo a couple of years ago and absolutely loved it. The snorkeling is very different from the Caribbean and the whole experience was amazing. In particular, Isla Espiritu Santo off of La Paz was one of the most amazing places we've been--snorkeling with sea lions and mobula rays. In your timeframe you might be able to snorkel with whale sharks in La Paz Bay as well.

Because we're not sailors I can't really advise you on your timing, but if you choose La Paz you'll need to remember that the Gulf of California is the Pacific and it's a lot colder than the Caribbean! Even over Memorial Day we needed shorty wetsuits at Espiritu Santo, although shore snorkeling wasn't too bad. It's windy over the winter and early spring (big kiteboarding place) and in the summer you get the Coromuel winds that cool things off on land but might make your overnights less than restful.

Lots of amazing food and drink out there--great seafood and tacos and margaritas--and spectacular scenery and wildlife both on land and in the water. I'd love to go back.