It depends on your outlook really. Spring breakers can really liven up a cruise ship. It can make for some pretty wild pool deck entertainment. Same goes for late May, right when school's out "for summer" (a little Alice Cooper ref for ya'll).
If you just want to get away and relax, then late January and February are good, as well as late September to early November (but then there is that darn hurricane possibility). I've been twice in October, with not a storm with 1000 miles.
I think the best month to cruise is any month that have a vowel in it's name...