You can also track the path of the boat, to figure out which port it came from and where it went afterwards. You can also correlate it with AIS, a mandatory tracking signal for ships (used for collision avoidance etc). Of course the ship could have turned it off, but if I owned a bunch of spy satellites I would pay very special attention to any ship that appears on satellite images but doesn't have a corresponding AIS beacon.

Of course unless you are tracking a submarine back to a submarine base all of this won't tell you exactly who it was. Any state actor can just rent a fishing ship and deploy a remote controlled submarine from it. That's where more traditional information gathering comes in.