AIS is an aide to navigational safety, but cannot and is not relied upon as a sole source of data. Not every vessel is required to have it, or have redundant systems for it. As this article suggests it's not verifiable information and can be spoofed in various ways, or just switched off if you're up to no good. Various international bodies are keen to come up with standards to reduce the opportunity for spoofing AIS, but it will likely not go anywhere.

The Project Sandstone series from RUSI digs into various techniques smugglers and sanctions busters use to hide or mask illicit activity at sea.