How to remove old anti-fouling from your vessel
To remove the anti-fouling, you need to get your vessel out of the water. Depending on its size, you can do this yourself or use a commercial slipway or dry dock.
Most small craft owners would first consider scraping and sanding the surface themselves. This method works well, but can be very exhausting. There is also the problem that the paint dust is actively toxic. You don't want to breathe it and you don't want to get it into the environment.
Chemical paint removers can save you some elbow grease, but they too involve a health hazard.
Abrasive blasting/sandblasting is both quick and effective, though that requires special equipment and expert operators. It's worth contacting commercial abrasive blasters near marine areas to find out what it would cost for them to remove the anti-foul. It will certainly save you time and trouble, and may cost less than you expect.
If you do decide to use a third-party contractor to remove the anti-foul, you may still want to organise it so you can personally inspect the hull, keel and any other underwater parts once it's been cleaned. You may spot blisters, cracks or other issues that need attention before it's all covered in a protective coating again.
When it comes to recoating your vessel, the universal rule of painting applies – surface preparation is crucial. The surface must be thoroughly clean and dry to allow the new anti-fouling paint to form the best possible bond. Not only the performance of your vessel, but potentially your life could depend on it.