Boat bottom paints come in single-season and multi-season types.
With single-season antifouling bottom paint, you have to apply it every season because the biocide loses its effectiveness after a single season.
Multi-season antifouling paints are hard or ablative. Hard bottom paint doesn't wear away as the boat moves through the water, the biocide just leaches out, leaving the hard coating. In high-fouling areas, you may need to reapply hard bottom paint after one season.