I quite like the marinetraffic.com site, but I've noticed some quirks. I don't know if they are bugs or side-effects of radio transmissions.
For example, I am tracking the cargo vessel ITALY EXPRESS. Its current position is steaming up the east coast of North America en route between the ports of New York and Halifax.
According to marintraffic.com, the most recent report of its position has been made by AIS station with ID 276, which according to the site is based in Norway! Station 226 "BARM" reportedly has radio capabilities of:- Max: 9.38 N miles, Average: 1.63 N miles, and coverage of 861 sq km, yet Olso (for instance) is 2700 nautical miles from New York.
Either station 276 is not in Norway, or the VHF AIS signal from ITALY EXPRESS is traveling a l-o-o-n-g way!