Catalina's waters offer some of the best snorkelling and diving accessible from mainland California, with kelp forests, clear water, and a variety of marine life including garibaldi fish (California's state marine fish), sheephead, calico bass, moray eels, sea lions, and the occasional leopard shark. Visibility of 10–20 metres is common on calm days.
Gear rental and guided snorkel tours are available from multiple operators along the Avalon waterfront. Lovers Cove, just east of the ferry pier, is the most accessible snorkel spot and is protected as a marine preserve — fish here are accustomed to people and will approach closely. For diving, Avalon Underwater Park (Casino Point) has permanent mooring buoys, dedicated dive entry stairs, and good conditions for both new and experienced divers.