From just about the time people started building boats, they've tried to keep the hulls foul-free. The fact is that many marine animals and plants consider boat hulls a perfect place to live. That, of course, makes it harder for your vessel to move through the water, so you need more and more energy for less and less movement.
Those early seafarers tried tar and oil to discourage hangers-on. They tried thin sheets of metal and then paints based on copper and tin. Some of these methods were not very effective or practical.
Anti-fouling paints have become the most popular method to keep boat hulls clean. These sacrificial coatings prevent or at least slow the growth of marine life on the hull and are meant to be removed from time to time to be replaced by a new coating.
By the 1960s, anti-fouling paints containing metallic compounds became most common. Unfortunately, they contained tribultyltin (TBT), which caused environmental damage around the world. New Zealand was one of many countries around the world that curbed the use of TBT.
The challenge for paint manufacturers since then has been to develop an anti-fouling paint that works as well as TBT, but without the harmful side effects. Most commonly, anti-fouling paint includes small amounts of copper to keep marine organisms at bay.
The type of anti-fouling paint that's best for you depends on the type of boat you have and where you use it.
Hard anti-fouling paints are good when going fast is what you want or if you plan to scrub your boat often. They can also be applied over old coatings. The downside is that once a vessel with this kind of anti-fouling has been out of the water for a length of time, the anti-foul loses its effectiveness.
Self-polishing (ablative) anti-fouling paint gets rubbed away with use, especially if you travel fast. The movement of water across the surface takes away the top layer, keeping a fresh layer of bio-active chemicals such as cuprous oxide on the surface to ward off marine life forms. Also, it doesn't lose its biocide potential if the vessel is out of the water and makes sanding and recoating easy. The downside is you need a thicker coat and it may not last as long as the harder anti-foul coatings. Also, the finish may not be as smooth.
Apart from efficiency, your budget is also a major factor in the paint you choose – especially since the copper content can make anti-fouling paints really expensive. It's probably best to discuss your requirements with a local expert.