All vessels over 300 tons and passenger vessels over 100 tons are required to carry an AIS transmitter. This broadcasts vessel data such as identification, location, speed and course on a VHF frequency. This is picked up by shore or vessel-based receivers and decoded into NMEA sentences. The data can then be used to map the vessel on a electronic chart or radar or combined with a receiving vessel's own location and course, in collision avoidance. AIS data may also be broadcast by or on behalf of static navigational aids like lighthouses and buoys.
Since the setup cost for an amateur shore station is minimal, anyone with line of sight of a busy stretch of water can set up their own. Some publish the results on the web.
A site which I came across tonight, http://www.aisliverpool.co.uk/index.php
is a wonderful example of what a enthusiastic web engineer can do with this data. No longer is that ship in the distance a grey blob - it's a vessel with a name, a speed, a destination, a closeup when mashed up with images from this site or http://www.vesseltracker.com/en/Home.html.
and possibly a story, a history of visits and voyages. In a small boat, that data broadcast to all and sundry could be life-or-death information to you. That distant blob on an apparent collision course is no longer anonymous, routeless and inhuman. If you are still uncertain about the ships intentions, it's so much less confusing to call up a vessel by name than some vague lat/long and bearing.
All this depends on the global unique, stable IMO number, introduced to improve the safety of shipping. On the web, it is this identifier which is the basis on any semantic web data and tools to bring this information together.
The problem for both the above sites is to garner a modicum of funds to support the engineer's passion. One key question for the semantic web is how to reward them for making their deep pot of information available as RDF. It would seem so wrong to scrape their pages, tempting though it is.