The Internet is a great way to learn about cruises (especially if you visit cruise blogs like this one!) and get a feel for what's out there and what price range various trips fall into, but it's not necessarily the cheapest way to book a cruise.
It's certainly the most convenient--you can book your whole trip without leaving your house or even calling anyone on the phone--but you can often find better deals elsewhere...
Believe it or not, travel agencies can often get you a better deal.
You'd think that because they're getting a percentage this wouldn't be the case, but agents are well-connected, often receiving daily faxes and e-mails from cruise lines and tour operators. This means access to deals that change faster than websites can.
In addition, a lot of travel agencies belong to travel consortiums such as Ensemble or Virtuoso. These guys book blocks of cabins to get cheaper group rates from the cruise lines, and then they pass along the savings to their clients.
Also, if you've cruised with a certain line before, you're often eligible for discounts because you're a repeat cruiser. It can be hard to figure out how to take advantage of that over the Internet, but your travel agent will know just who to call.
So, just because websites promise great deals on last minute cruises, don't assume they're listing the best possible deals out there for everyone. Often, those who plan ahead and book with an agent can get even better deals than folks who book online!