Robin has a very good point in constricted areas such as harbors. However I believe that there is a good arguement for AIS both in passagemaking and on the ICW in some places. Many times recreational vessels are not seen--or if they are seen they may be items of curiosity. AIS at sea will show your vessel long before it may be seen on radar. I have had several instances where I had to take emergency evasive measures to avoid being run down by a commercial ship. Usually these involved difficult visability. For example just before entering the Straits of Messena --there are multiple shore lights, and also multiple radar targets. A coastal freighter came around the corner--all be it on the wrong side of the channel--and only the reflex of both of us turning hard to srarboard avoided a bad outcome. (That was a congested area, where AIS would be of benefit for the rare recreational vessel). Another instance is on the some ICW sections, where you cannot see what is around the bends--either visually or with Radar--the AIS gives you this vision--for both the commercial and the recreational vessel. As for San Franscisco bay--I agree that hundreds of recreational vessels, would be a problem, but AIS B can be filtered out. Bob Austin