I have found that cruise prices are like buying a car, they change constantly, take your time, research, research and research. Stay with reputable travel companies or agents and let them work out a good deal that fits you and your price range. Do not get in a rush and speak with several agents/companies to get the right price, upgrades and other perks.

I only think booking a cruise while aboard is worthwhile if it's under a system like the Princess Future Cruise Credits. You don't have to commit to a date/sailing, you can transfer to your TA, reduced deposit, OBC based on # of cruise days, and an expiration date of 4 years. Pretty good deal IMHO. Actually booking the cruise onboard is too much of a hassle, I'd rather shop around for the best price.