I heave-to past the 3 mile limit to scrub the bottom of my boat, but I don't do it for environmental reasons. I do it because of the gators.
All copper-based anti foulings (abaltive or otherwise) leach copper throughout the course of their useful lifespans whether the boat is in motion or not. Further, most boats spend the vast majority of their lives sitting in a slip in a marina, doing little more than leaching copper into the water. There can be little doubt as to why there are elevated levels of cuprous oxide measured in virtually every marina that has been tested for it.It is definitely a question of dilution. True, ablative paint wears away slowly as the boat travels but the amount is miniscule compared to what is concentrated by pressure washing, scrubbing or sanding. Natural ablation also tends to occur in deeper, open water where it can be further diluted and carried about.