I got hooked on AIS by my brother-in-law, a ship pilot in Aransas Pass/Corpus Christi. I won't leave home overnight without it. The free standing unit will cost about $400, and needs
GPS and
VHF antennas. Mine are dedicated, but I think you can share. It gives you a
screen with other boat/ship symbols and nav markers, tells you who to call and them who you are, and calculates
collision and nearest point of approach for you - sort of a super
radar. My B in L kept sending us
Google Earth screen shots with our
boat marked on our last
cruise. The nearby stuff is done by
VHF, but the information also goes by
satellite, so if you want an idea,
Google "AIS map." My only objection has been
boats leaving them turned on while in
marinas, so I get a bunch of
collision alarms when passing by in the channel.