Skimmer (spa)
The density of fresh water is 1,000 kilograms per cubic meter, while the density of seawater varies between 1,020 and 1,030 kilograms per cubic meter.  Oil is less dense than fresh water and seawater, so it floats in both types of water,  but due to the difference in density the oil and fat particles, dispersed in the water, will reach the surface more quickly in a saltwater or seawater pool,  being in both cases, "the only possible means" to remove them, to recirculate the water surface through skimmers .