I can be within range of several hundred boats at once... and the percentage of AIS transmitters is probably .001% or less. I suspect folks just haven't rushed out to fit AIS transceiver's just because they exist...
But then also the Marine Traffic system does depend somewhat on volunteer receivers who retransmit AIS data via Internet to the Marine Traffic site. There are also now some satellite AIS receivers feeding the Marine Traffic site, but I don't think they pick up and relay every AIS transmission out there...
South River, Chesapeake Bay