Duckweed is the ultimate floating plant. It propagates quickly and is extremely easy. Duckweed prefers a still surface to grow on.
This is a great floating plant if you have an algae problem or high nitrates.  Duckweed floats at the surface, blocking out light, but is a fast grower and will consume excess nutrients in the water.  
(A portion is around the size of a golf ball clump)