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This has been dodged continously in the story of One Piece. The One Piece may not be tresure but I think several keys for example several different things. I think part of this is a Devil Fruit called the Mizu Mizu no Mi (Meaning Water-Water Fruit).

This element would be extremly strong because, water changes the rules of devil fruit powers. The "All Blue" may be connected to this as well both being myths. The Rio Poneglyph may be included as a tresure in the One Piece.