The main benefit of Cruise Shark covering these cruise lines is that they have a price matching policy where if they drop the price of your cruise after you have booked it they will reduce your price accordingly (subject to terms and conditions). The cruise lines operate this policy on the basis that it is not done automatically, you have to monitor this yourself and bring it to their attention. Cruise Shark will let you know if the price drops without causing too much hastle and could save a lot of money, it has certainly saved the person that brought this to my attention hundreds of pounds. This software is hosted by www.cruisenotes.com and you will need to register on their site before being able to download Cruise Shark.
You can get Cruise Shark from here http://www.cruisenotes.com/