No entirely true: A lot of information is transmitted via AIS these days. For example current water depths (St Lawrence Seaway or on the Danube in Europe) are transmitted and used for navigation on these waterways. You also have the possibility to place a distress signal with AIS, which would quite likely lead to a Search and Rescue operation, costing huge amounts of money and taking away resources from real emergencies.

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.