Plankton Crab larvae. Try taking another look. It's probably inhabited by millions of tiny plants and animals. Some of them you might be able to see floating around if you scooped up some of the water in a glass jar, and some you might need a microscope to see.
Plankton includes plants and animals that float along at the mercy of the sea's tides and currents.
Others will remain plankton for their entire lives. All jellyfish, and the Ocean sunfish are such feeble swimmers that they too are included as plankton. Most of the plankton in the ocean are plants. Phytoplankton produce their own food by lassoing the energy of the sun in a process called photosynthesis. So for sunlight to reach them, they need to be near the top layer of the ocean.