The Moon Turret (「月見櫓」?) can be found at Ryoshima Coast after restoring the Guardian Sapling there. Its only purpose in the plot is revealing the entrance to the Sunken Ship. After learning Holy Eagle, Amaterasu can jump onto the turret and use Crescent in the sky. This causes the water in the Lunar Lagoon to recede, making it safe to walk on the ocean floor and into the ship.
The Sunken Ship will become inaccessible to Amaterasu again when dawn breaks at the end of her first visit there with Rao. She can return to the ship anytime afterward by jumping back up to the turret and using Crescent again.