The point is that you only have to run your boat through a relatively small patch of oil to contaminate your antifouling paint. Once that happens, the biocide in the paint is prevented from being released. That means the hull will be quickly fouled with marine growth, which increases the hull’s resistance going through the water, which drives your fuel bill up.
In addition, you’ll have a contaminated layer of petroleum on the hull that, if it stays too long, makes it very difficult to apply new antifouling paint. And if crude oil remains on paint above the waterline, the paint will be stained and degraded.
All of this is pointed out in a short brochure from Interlux and Awlgrip, along with some pointers on what needs to be done.
For heavily contaminated (thick, sticky, tar-like) hard-polishing and ablative antifouling paints you are going to have to use a paint stripper to get rid of the oil and the old paint. Then you will have to scrub the cleaned-off surface and wash it down with water. If the hull is just lightly contaminated, a good power washing might do the trick, followed by a scrubbing with Fiberglass Surface Prep YMA601.
The above is for fiberglass boats. On the bottoms of metal boats you’ll have to use a grinder or sandblast the surface after it’s cleaned off. If you don’t clean the surface before sandblasting you will just drive the oil into the metal, making it hard for antifouling paint to adhere to the hull.
That’s pretty much the gist of the brochure. For more details you probably should contact your paint supplier.