Hi there Jeanie,
If it were up to me, I would choose either February or November, as March begins to see an increase in crowds due to many Canadian schools observing March Break. I have actually been able to secure some of the best package deals in February do to it being an off-peak month, but it's really hard to predict when Disney will announce promotions and what said promotions will entail.
If swimming is important to you then November would be better than February, as February's temperatures can still be a bit brisk. I was at Walt Disney World last week, and in December, the temperatures were in the high 70's and plenty of people were relaxing in the pools!
Another bonus to November, in my opinion, is the Holiday decor and special events. The Holiday season has quickly become one of my favorite times to visit Walt Disney World! The downside to visiting in November is that you'll have to wait almost an entire year for your next vacation! How about February AND November? ;)
All of that said, I highly recommend keeping a close eye on the
Special Offers page of the Walt Disney World website as special offers and promotional deals are announced frequently throughout the year!
Hope this helps, and please feel free to stop back by with any additional questions!