I work for a consultancy that does vessel tracking as one of its main products, and yeah it's expensive! afaik they have remote teams with sensors at key points and a bunch of people using AI/software to manage things like GPS spoofing. So it's all pretty guarded proprietary stuff.

Great bit of topical datavis here.

Yeah! The AIS terrain data is expensive, but the good stuff is from satellite tracking and out of my budget for silly site I built on a whim.

https://i.imgflip.com/aopmmf.jpg