Siete Pecados — "Seven Sins" — is a cluster of seven small limestone islets just outside Coron town, protected as a community marine reserve. The shallow coral reefs that fringe them are the most accessible snorkelling site in the area: most island-hopping boats stop here for an hour, and the water is shallow enough that even non-swimmers can drift with the current and see the reef from the surface.
Expect to see clouds of damselfish and chromis above hard and soft corals, the occasional clownfish in its anemone, and turtles fairly regularly. There is a small entrance fee that funds the reserve.