The shaded areas are where there is AIS
coverage and ships have recently been reported. The marinetraffic.com site is fed from many different AIS receivers, usually run by enthusiasts (I am one of these), perhaps locally aggregated, then sent over the internet
to their site for display. My AIS receiver is in my barn, overlooking the Pacific north of San Francisco
at an elevation of 1000 ft, and I am feeding data to several different networks. There are many other receiver sites in the S.F. Bay area, but (for example) there aren't any between my house and Oregon
, so there will be no coverage along that stretch of coast. You will not see much traffic out to sea, since these are land-based receivers and AIS reception
is usually line-of-sight. There is work with commercial satellite reception
of AIS, but as far as I know this is still at the experimental stage.
There are other loosely organized AIS receiver networks (shipplotter, aishub.net), and some commercial
ones as well. This is fascinating stuff.