Here is a quick video showing how to build a floating deck economically with used pallets.
*This deck is meant to be temporary (12 years max). Could be a functional addition in a backyard at a rental property, etc...any situation that is meant to be temporary. Using a water seal or paint on the pallets underneath would help some with durability.
For a more durable deck, one could use treated wood to build pallets or use a floor joist type construction method with treated wood.
You would also want to apply plastic or landscaping mesh/fabric to prevent grass from growing under the deck.