Moving fouled boats only applies if you then clean the hull in the new location. Marine fouling stays put on the hull.
Sure, the bloody things reproduce, but they do it by laying eggs in the water, so on your hull or the marina, it's the same. Clearly I don't pick up organisms when I'm out sailing, so I get infected from the marina itself. And some marinas have the cheek to say that you cannot clean your hull in the marina. Its" please go elsewhere to do it, and spread the stuff even further"