This may not look like much but if you follow my advice it could save you hours. It is in fact abrasive mesh designed for sanding down drywall but I found some years ago that it works a treat for prepping anti fouling ready for re coating.
One of the least popular jobs amongst do it yourself owners is probably painting the bottom of the boat each year. Often working overhead or lying on your back in a cold and windy boatyard the paint drips on everything but before you even get to that unpleasant stage you have to prepare the surface. Anti fouling paint is toxic and dry sanding sends clouds of dust in all directions, coats everything nearby and is a serious health hazard. Wet sanding is safer, produces no dust and with drywall sanding mesh is about 5 times faster than normal abrasives and is a system that I have been personally using for many years.
After power washing the boat and getting off all the big lumps pop down to the local home store and buy yourself a hand drywall sander and some 120 grit sanding mesh. The sander and mesh will cost about $10 in total. Get yourself a bucket of warm water and dip the sander into the water then while still wet start sanding the anti fouling until smooth ready for the new paint. The mesh cuts fast and because it is made from fiberglass with grit impregnated into it, it is water proof and does not clog. The secret is to keep the mesh wet, you will feel as the mesh starts to dry, the sander will drag and slow you down. Run your fingers over the surface to check for smoothness as you go. I have to warn you that the water will run down your arms so now you know why I suggested warm water, it doesn't work any better it is just slightly more comfortable as it dribbles down inside your shirt. You could also use a spray bottle to keep the surface wet as you go but I do feel that you will be pleasantly surprised how fast the jobs goes. The last time I used this sanding method it took me about 2 hours to sand a 30 foot sailboat and I only used two mesh sheets.
I thought I had a picture somewhere of me using the sander but I can't find it right now so by way of a consolation here is a shot of a chap sanding drywall so you get an idea of what a hand sander looks like and what you should be looking for in the DIY shop.