Concha de Perla is the simplest snorkelling on Isabela: follow the boardwalk from the harbour and drop into a sheltered natural pool. Sea lions are the usual stars, and the calm water makes it a good first Galapagos snorkel before booking longer boat trips.
Go early or late for fewer people and softer light. As everywhere in the Galapagos, keep your distance even when the animals seem completely unconcerned.