Looking at the dates of your past cruises, you should be able to answer this question better than most.
But there are several "off-seasons" during the year. In fact, there are more off-season times than on-season times. And rates will always reflect when it's on-season and off season. So let's look at when the on-seasons are and the rest are off-season.
I'll be speaking generically because there are always exceptions to every rule and some areas are affected by off-season different than others. Basically, the on-season is anytime school is out of session. Christmas/New Years is the most expensive time of the year, followed by Spring Break, Thanksgiving, and the first several weeks of Summer vacation. Anytime outside of those 4 timeframes is generally off-season.
The 2nd/3rd week in September is one of the cheapest times of the year, pretty much most places you'd like to go, including Alaska and Europe. But it's especially true for the Caribbean because it's also the middle of hurricane season, which scares alot of people. This is when we like to travel because we get the best rates. Another really good time to cruise is the 1st/2nd week of December because people want to travel for Thanksgiving and Christmas, so they're not thinking about traveling in between. Same holds true for the weeks following New Years.
As for what the crowd is like or whether it's a good or bad experience, we've traveled in just about every month and to us there's really no different than any other time of the year except there are fewer children. If anything, maybe people are more relaxed because there are fewer children. Often in the middle of Winter you'll find more people from the northern States and Canada, but that's neither a good thing or bad thing.