Minecraft wiki entry for Secret_door, associated text: Since the player's visibility is limited underwater, an underwater entrance is easier to conceal. In fact, most players will not notice a 1 block hole underwater unless they are looking for something like that. Simply dig a 1 block hole from your hidden location to the water. For extra security, it can be coupled with a piston and an opening mechanism. One simple mechanism for underwater is to use water stream to push an item onto a wooden pressure plate. It is better to have a separate entrance for the player and item so that the player cannot press the pressure plate themselves. A more simple method is to cover the entrance with kelp and/or seagrass and a warped trapdoor (as the green color blends in the water well).