The mussels came from the Ukraine in ships ballast water to the Greart Lakes a number of years ago. There has been a concern about Western Lakes being poluted the Quagga and Zebra Muscles, and any Eastern Boats, and if the boat had been in potentionally contaminated waters in the last month, boats launched at Lake Powell, were required to have the bottom hot pressure washed. Jan 2007 the mussels were found in Lake Meade. This January the muscles were found in San Justo Reservoir in San Benito County, California... Additionally Lake Mohave, Lake
Havasu, and the Colorado River Drainage below these lakes have been known to be infected. There were some Larvae found last summer in Powell. The muscles grow very prolifercately and plug up water intakes and wreck damage on the canals and pumping stations. They also are filter feeders and can alter the food supplies on the affected lakes.
The bottom wash is somewhat misguided, since thearval form can exist in bilge water--may actually be a better vector of transmission that the external part of the boat. Also bait tanks, engine cooling systems, waders, trailer box frames, and ropes etc. Dry heat and letting the boat dry well will prevent growth of the mussels. High pressure washing on raw gelcoat in the 3000 to 3500 PSI range can damage the surface of the gel coat. If the boat is bottom painted, there is some damage to the bottom paint. The older the paint the more damage..However, it is routine to pressure wash the hulls of boats which are hauled for bottom painting.
I personally do not pressure wash my boats, with one exception--that is an old dinghy which has some chalking of the gelcoat already. If there is any grit of debris on the surface of the boat, the pressure washing may cause some abrasion., The hot water is not a probelm--if the pressure high, the chance of damage is increased. I don't think you have any choice--the rules are in place and if you want to use the boat it will have to be washed.