The underwater one is the one associated with the Perpetual Machine Array boss, actually. And, it's tied to a series of rather annoying puzzles & part of the Sakura Cleansing. You have to do some blocks in a very specific order to drain an entire thing of water, and then another set of Electro Cube puzzles to do the same thing.
Had to Google it in order to get that one. The waypoints I'm missing are two domains.
Finishing Orobashi's Legacy unlocks the Formation Estate Domain in Yashiori Island. Because... the thunderstorm doesn't clear until you finish that (annoying) questline.
I'm also missing the domain in Watatsumi Island, but I forget how to do the puzzle.
Had to Google it in order to get that one. The waypoints I'm missing are two domains.
Finishing Orobashi's Legacy unlocks the Formation Estate Domain in Yashiori Island. Because... the thunderstorm doesn't clear until you finish that (annoying) questline.
I'm also missing the domain in Watatsumi Island, but I forget how to do the puzzle.