Snorkeling and scuba diving are possible during the clear-water December–April season. That’s when California-based scuba instructor Douglas Storms (Juárez 187B, tel. 323/285-1418, www.divingbeyond.com [1]) offers snorkel and scuba lessons and excursions out of his Adventure Center headquarters, a block from the Estero El Pozo dock. Off season, you can contact him in Sausalito [2], California (tel. 415/331-7925 or 415/325-3789).
Links:
[1] http://www.divingbeyond.com
[2] http://www.moon.com/destinations/california/san-francisco-and-the-bay-area/north-bay/sausalito-and-tiburon