The frequent guest programs of the lines aren't really 'linked', but once you have been on one you do get a few specials on the others. By proving (faxing) that I had previously been on a Carnival cruise I was able to book the least expensive balcony and was immediately upgraded to the most expensive balcony for free. Here is the link to the information...

http://vip.worldsleadingcruiselines.com/vip/