It is a backup system in case the primary electronic weight control system fails. The tethers that hold the weights in place dissolve in ocean water after 24 hours and drop the weights.
I don’t know how you could possibly improvise on the ocean floor and use “hydraulics” to drop the weights.
If that’s the best technology they could come up with they had no business doing what were doing.......good thing there weren’t any 50 year old White guys involved.