Skimmers are machines like vacuum cleaners that pull up the oil from the surface of the water. The water must be calm in order for skimmers to be effective.
Skimmers are similar to vacuum cleaners, but instead of sucking up dirt from hard surfaces, they suck up oil from the surface of water. The water needs to be calm in order for skimmers to be effective.
Your own original version is better than any of the commenters' suggestions. Nothing wrong with the original at all.
Skimmers are machines - like vacuum cleaners that effectively pull up the oil from the surface of calm water.
