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.
If the ship is small enough, it wouldn't be required to have an AIS beacon. From an article someone else posted, the suspected ship is a 50-foot recreational sailboat.
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.