Portuguese man o' war
The Atlantic Portuguese man o' war lives at the surface of the ocean. The gas-filled bladder, or pneumatophore, remains at the surface, while the remainder is submerged. As the Portuguese man o' war has no means of propulsion, they move according to the winds, currents, and tides. Although they are most commonly found in the open ocean in tropical and subtropical regions, they have been found as far north as the Bay of Fundy, Cape Breton and the Hebrides.
H. G. Wells' War of the Worlds (2005 direct-to-video film)
The war machines are crab-like "walkers" with six legs. A Heat Ray is built into the machine's "head", and is fired from a single eye. The fighting machines do not appear to have protection against modern artillery (avoiding the "invisible shields" seen in the 1953 film version and Steven Spielberg's 2005 film), leaving their ability to conquer unexplained. The aliens do have a substance similar to the black smoke, but is more of a dense green toxic gas unable to rise above ground level, allowing survivors to escape by getting to high places.