Imagine not using an antifouling on a yacht. Over a short period of time there would be a build up of slime, algae, shell fouling etc leading to a very rough and heavy hull. If motoring, in order to maintain the previous cruising speeds, it would require more power from the engines due to the extra weight and drag from the fouling. This in turn increases the consumption of oil and thus emits more waste gases. It has been proven that a fouled vessel burns 40% more fuel and emits 40% more waste gasses (CO2, SOx) increasing air pollution.
Using an antifouling will eliminate this hull roughness, reducing the drag caused by fouling and, according to figures produced by the American Chemistry Council, will save ~200 million tonnes of CO2 and ~5 million tonnes of SO2 being emitted annually.