There are several, but I would recommend you the one I use each day - at shipcruise.org. Their cruise tracker not only shows the ships' current positions, but this feature there is combined with the ship's cruise schedule as departure dates and min prices - I use it bcoz of my work. At shipcruise they update the cruise dates and prices info each month. This is the link to their tracker
http://www.shipcruise.org/live-cruis...acker-locator/
It's very easy to find there the ship and track its current location - they all are listed as links. It also has info on how the AIS tracking works - of cruise ships and of all marine vessels in general - they all got this position transmitting GPS-like equipment. The shipcruise tracker doesn't include river ships, and also some of the smaller luxury ships.
Btw, as AIS data they use the vesselfinder AIS server - this is also a very good ships tracking site, but there cruise ships are not grouped separately, so there you have to search by name or imo number - so it's not a cruise ship tracker per se. This is the link for the vesselfinder tracker
http://www.vesselfinder.com/