Booking at the wrong times
If you don't have kids, and book during the holidays, early April, or summer vacation, guess what your cruise is going to have a ton of? Not hot single people looking for weekend flings, that's for sure. Similarly, if you're looking for a family cruise and book during March (aka college spring break) prepare to explain to your 6-year-old what a beer bong is. And weekend cruises? Those are big floating trips to Vegas that won't have you leaving feeling refreshed.
Along the same lines, remember that sometimes cruises offer cheaper fares when the weather is terrible. While no sane person is taking an Alaska cruise in January, booking for the Caribbean in July could well have you looking at overcast skies and rain all day. Or, worse, get you caught in some nasty tropical depressions.