An ocean reef
Similar to Woods, Reefs provide some degree of defense to Naval Units, at the cost of being slower to move through. Note that Reefs only provide 10% additional defense, while Woods provide 20%.
Like Woods, Reefs will hide any Naval unit within it in a Fog of War game, only being revealed if the enemy has a unit adjacent to it. Like Woods, this feature does not extend to Air units; they will be seen as normal if they are flying over a Reef.