Select a wire basket suited to the size of the body of water you wish to float a planter in and the plant you'd like to grow. Pan-like baskets work well for this project. You can also use a solid, lightweight copper or aluminum pan for a floating planter.
Cut expandable polystyrene foam, or EPS foam, to the size of the wire basket. EPS foam is the buoyant white foam that disposable coolers are made of. You can find blocks of EPS foam at most craft stores. Using multiple pieces of foam works too, as long as it is tightly packed. Cover the entire bottom layer of the basket with foam, leaving about a third of the basket's depth for potting soil.
Fill the remaining third of the basket or pan with potting soil.
Add seeds or transfer a young plant to the potting soil.
Water your seeds or plant and float your new planter in a body of water.