Marine vessels around the world use the Automatic Identification System (AIS) to send their position to other nearby ships and AIS base stations. The use of AIS allows websites like MarineTracking to map the position of ships across the globe.
If you want to create your map of marine traffic around the world you can use the OWM AIS API. As well as providing access to AIS data OWM AIS also have a neat a heat map view of all ship traffic around the world.
The map allows you to see the busiest shipping lanes across the globe. The map allows you to select from different types of marine vessels. This means you can view heat maps for passenger ships, cargo vessels, fishing ships and even pleasure craft. Source