While this may seem like a strange question there are in fact many reasons why using an antifouling is actually good for the environment. 

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, SO_x) 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. 

As well as the elimination of waste gases, antifoulings also aid the environment in other ways; 90% of the world’s cargo is transported by sea. There are also thousands of pleasure craft that circumnavigate the globe annually. If these vessels did not use antifouling a significant proportion of them would quickly become heavily fouled. Consider that any number of these vessels could now journey to various oceans and rivers across the globe and it is easy to see how invasive species (aggressive non-native species) can be transported easily from one area to another on the bottom of hulls of boats. A fouled vessel can transport these unwanted invasive species into sensitive ecosystems, resulting in catastrophic damage. Using antifoulings prevents this from occurring and protects our sensitive ecosystems. 

Antifoulings also help reduce pollution. How? A fouled vessel will be forced to dry-dock prematurely for cleaning, re-painting, repairs etc and this process will inevitably lead to an increase in pollution from boatyards. Using an antifouling can lengthen the time between dry-docking, minimizing the pollution resulting from those dockings.