Here's a cautionary note from an article about a trawler/sailboat collision off Oz:
Here's a link to the story:
http://www.sail-world.com/cruising/index.cfm?nid=59302&rid=11
It is also worth noting that dependence on an AIS system will not guard against collision with fishing vessels, which are not currently covered by AIS regulations. There are also some jurisdictions in the world under which it is not necessary for ships to carry an AIS transponder. For safe ocean travel by a yacht, it is advisable that a visual human watch should still be maintained at all times, using an AIS as supplementary aid.
Here's a link to the story:
http://www.sail-world.com/cruising/index.cfm?nid=59302&rid=11