One site that I've always enjoyed looking at is cruises.com. I recommend getting on their email list if you are thinking of vacationing soon.
They have discounted cruises, last-minute cruises, or short-term sales (blast sales that are only a few days long). Some of their cruise prices are amazing-ridiculous! In fact, before I knew I was pregnant, my husband and I almost booked a 5-day trip to Mexico. It was only going to cost us about $230 each. NUTS!
Have fun cruise shopping!