They arent, however the rules of the sea are very simple. More importantly a calculation of probability of collision between two slow moving giant metal objects would be easy.

Ever been a skipper manoeuvring in confined waters with multiple vessels around you? I have, and it is at least an order of magnitude more complex than anything I've ever encountered on the road.

Still; assuming you can get good, real-time estimates of mass, rough shape, position and velocity vector of each of the ships around you, making a system to compute probable collisions sounds like a small undergrad-level project at best. What are the obvious things I'm missing here?

You would need to account for drag and windage. Basically know everyone's hull shape and CFD it! And the seabed under them.

At a distance it's easy - but in confined waters it gets very complicated very quickly. I've seen situations where literally the only solution is, who do I collide with.

In the case of the USN they have a hell of a lot more manoeuvrability than a freighter but they can't escape the laws o'physics...