AIS receivers are still a bit expensive but hopefully they will come down in price with popularity.
An AIS receiver receives signals from commercial ships that are required to carry AIS transpnders. The ships continuously transmit ship's name, call sign, vessel type, speed, heading, course, position, navigational status and MMSI number for DSC calling.
The receiver outputs NMEA -0183 to your chartplotter or radar(if compatible) and will be displayed on the screen with the ships information. This allows you to see, graphically the direction and course of the ship and enables you to call the ship by name on the VHF, or even directly, using a DSC VHF.
The AIS receiver operates at VHF frequencies 161.975 (Marine ch 87) and 162.025 (ch 88) MHz (9600 bps GMSK 25 or 12.5 kHz HDLC). Unfortunately, the audio signal one gets from a normal VHF cannot be used for AIS data. Apparently certain model Icom VHFs can be modified for this purpose, Hopefully, one day AIS will be incorporated into all our standard VHF radios.
The deluxe version is an AIS transponder, that will not only receive the information, but also transmit your vessels information to other ships in your vicinity.Howard [ Just Imagine ] 13-Mar-2008http://www.waypoints.com/ais.html
Smart Radio SR161 $189
Conflicting opinions about Nasa/Sitex low cost AIS receiver
Comparison $200-$600 from MilltechHoward [ Just Imagine ] 13-Mar-2008