While boaters and fishermen may help spread these invasions, they are generally started in other ways, large ships being the most effective means of contamination. Ballast water is taken on after unloading at some port, and emptied when cargo is laded at another. I seem to recall that of the 200-odd marine invasive species in SF Bay, they are surmising that about 80% were transported by shipping.
Kind of a moot point once they're in place, I suppose, but at least they're aware of it now. There is a movement afoot to have ballast water dumped and replaced at sea to minimize new infestations. Also sterilization of bilge areas, but that's not likely to go anywhere.
Peter_C probably has more info (as he does more fresh water boating than I do), but we now have mandatory inspections at a lot of lakes to prevent Zebra and Quagga Mussel infestations (I have never had my boat inspected.) I was actually kind of wondering how they inspect in the rain since they're looking for a dry bilge area (among other things.)