Vessel tracking systems have been dramatically changed after the implementation of Automated Identification System (AIS). AIS became mandatory on all ships above 300GT, passenger/cruise ships and vessels carrying hazardous materials.
The low cost of AIS-receiving equipment ($200-$500) helped huge number of radio-amateurs, ship spotters and enthusiasts to purchase and install home-based AIS stations. Almost all of them stream their data to one or more web based vessel trackers or just exchange their data for other feeds.
Live cruise ship tracking is possible because all passenger cruise ships are equipped with AIS transponders. When the cruise ship is in the zone of AIS coverage, you may track it via one of the listed below free ship tracker portals.
1) VesselFinder - according to users' reviews online - it has the most easy-to-use interface, it provides very good coverage and data reliability, it's constantly developed, it has a great ship news section, it's fast, it's free.
2) ShipTracking EU
3) SiiTech COM
4) Digital-Seas COM
For professional ship tracking there are several systems, providing better quality, support and more specialized features (ordered by price):
1) VT Explorer (~480 EUR/year)
2) VesselTracker COM (at around 500 euro/year)
3) AISLive COM (charging around 1000 euro per year)