I recently had a fellow sailor - who is just IN LOVE with his iPhone - show me an app that displays vessel positions received via AIS. He said: "Why should I buy an expensive receiver and chartplotter when I have this
Since there may be others who are tempted to forgo the expense and use their iPhone or other mobile device instead of a real AIS receiver and traffic display, here're the answers to that question:
1) The AIS data received on your iPhone, laptop, via the Internet, or by any source other than an actual AIS receiver located in your vessel is posted to the Internet by volunteers who have receivers located in fixed positions (often far from water). I'm one of those volunteers. The vessels you see near Half Moon Bay, CA on marinetraffic.com and aprs.fi probably came from me (look for "N8QH" as the source). My receiver is in my sailboat, from which I upload a continual data stream to marinetraffic.com and aprs.fi via the Internet. And when my boat leaves the dock, I disconnect from the Internet and ... POOF! ... all of the data I am supplying, and seemingly all the AIS-equipped vessels in that area, VANISH. There are no other volunteer receivers that cover the area because the surrounding mountains block reception. This is true for many other volunteer receivers: their receivers are only on part-time.
2) If you think you can use AIS data from a service via your iPhone for collision avoidance, then read the fine print. Here's an example warning posted on marinetraffic.com:
"Vessel positions may be up to one hour old or incomplete. Data is provided for informational reasons only and is not related by any means to the safety of navigation."
The author of that warning isn't exaggerating or just trying to avoid some obscure legal liability. For the SF Bay Area, there are only a handful of volunteer receiving stations, and the coverage in the Bay is highly incomplete. Large swaths of the Pacific Coast (and most of the remainder of the planet) have no coverage at all. If you don't see any vessels on your iPhone AIS app - that doesn't mean there aren't any vessels there.
So don't cheap out by trying to use your iPhone along with some external service to avoid getting crunched by a freighter! The data is unreliable! It's false security. The only AIS data you can trust is the data received by an AIS receiver located in your own boat.