These arch shaped keys have a structure similar to that of an atoll within the archipelago, they are located in the southern limit of the national park less than an hour and a half in boat from Gran Roque.
Cayo de Agua takes its name from the wells of fresh water that are found in the inside part of the key next to a group of palm trees. When the water plants for the processing of sea water did not exist, these wells offered an alternative for the supply of drinking water. Some of the inhabitants explain the existence of these water wells as the result of the thickness and breadth of the sand layer that covers the key. This layer works as a sponge that stores rain water; the fresh water absorbed by the sand forms a bag that does not mix with sea water due to the differences in density. In this way the fresh water remains in these artificial wells.