Putting floats to hold the top of the plastic sheet above the water, would do the same, and allow the sheet to move, as the water pressure/ level changes.
The idea is to create a boat shaped tube, (from the boat to the water surface), which water can be removed from, allowing, in essence for the water under the boat to push it up!
Hence the need for big pumps this water has to be removed quickly, before more leaks in, which it will do till the sound hull is empty.
Bod
Experiment.
Bucket of clean water, beer/milk bottle, and drinking straw.
Put enough water in the bottle to just float it with the neck above the water in the bucket.
Suck out some of the water from the bottle, the bottle will rise.