Its bound by Solas regulations that are legally enforceable. Ships must have two way AIS and its logged, so they know if you were in range similar to the tracking for planes. Boats under a certain size do not need AIS and often only have one way AIS, but I expect thats not the case with DiCaprios boat.

Solas was bought in because of the Titanic sinking as other boats declined the help due to the risk of Icebergs, so thats a further link to DiCaprio.