Hey...I just ran across this neat AIS receiver that is cheap enough ($189) for just about any boat. A nice addition that can tell you what big ships are headed your way and whether they will hit you! Note that this does not give YOU an AIS identity but lets you call the ship you are worried about by name and get their attention. Pretty cool.
http://www.milltechmarine.com/SR161.htm
http://www.milltechmarine.com/FAQ.htm