Anything that can float on the surface of water is a floater. When you relax in a pool, floating on your back, you're a floater.
If you throw pennies in the water, they'll sink, but a dollar bill will be a floater, as are dragonflies on a lake's surface and beach balls bobbing in the surf. A completely different floater is a tiny speck that seems to float across your vision, a small, nearly-translucent deposit on your eyeball of the sort that becomes fairly common as you get older. In US police slang, a floater is a body found in the water.