It was some kind of sluice gate. But the water level was all wrong.

Delighted to report that there is a DOUBLE SLUICE in the new Legend of Zelda: Tears of the Kingdom! A floating island in the South Hyrule Sky Archipelago features a sluice which feeds into a second sluice, each of which can be both raised and lowered at will, revealing a treasure chest at the bottom of each chamber.

@SluiceHype <3