(work in progress)
The machines that make plastic bags for potato chips, etc. can easily be adapted to create a long string of bags filled with air. When these strings are sealed together, you get a floating air bed in any size at extremely low cost ($0.009 / sqft, 400$/acre).
A similar idea is here: http://www.geocities.com/vacoyecology/Bubble_ponds_fluke_boats.html.
Soil would make it expensive. If the bags are not completely filled with air, a couple of layers may damp small waves.
(See also Stirling engine)
Amtec has seal that costs about $5 / 100 m2, but we need twice that so it's about $100.000/km2, $400/acre, $0.009/sqft.
Acts as a breakwater, not sure how much this effect would be.
But easy to renew parts of the land.
Biodegrading takes centuries, but according to Wikipedia Daniel Burd, a 16 year old Canadian discovered in May 2008 that Sphingomonas, a type of bacteria, can degrade over 40% of the weight of plastic bags in less than three months. "The applicability of this finding is still a matter for the future."
- Polypropylene can be recycled easily. Because it's one big plastic structure, it's probably easy to take it out the water, compress it and bring it to a recycling company.