AIS is not used for navigation, or any critical decisions. It's not like it will override what the bridge sees, or what the radar picks up. ADS-B is a similar system for aircrafts, and have the same shortcomings.
AIS is directly connected to an ECDIS on a ship's bridge, which is the digital replacement for maritime paper charts. AIS targets are displayed in these ECDIS systems and (see above) in some regions of the world the information shown there is also influenced by AIS data.
Also a lot of ports are using AIS (together with radar) to keep an eye on the traffic - spamming those systems, which is easily possible, would quite likely cause severe troubles for larger ports like London or Los Angeles.
I'm honestly surprised that nobody has yet DoS'd a larger port or other infrastructure.
It's probably because most people don't want to do that. Outside of prisons, a good chunk of human safety relies on the fact that most people would rather preserve their own safety than take away that of others.
Also, the AIS data is self reporting so if they forget to change the information then it's useless anyways. I've seen 'anchored' ships driving away at 10 knots.
> AIS is not used for navigation,
> or any critical decisions.
It worries me.