If you are near an airport that Southwest uses, I highly recommend them. I've never had a flight cancelled, only delayed twice ever, once for mechanical reasons, once weather related (New England). They are pretty good about sticking to their schedules. I've flown Delta and USAir several times and almost always my flights get changed. Another plus for Southwest is that if you do have to change planes, you most likely will be in the same terminal for each. Nothing worse than having to get from one terminal to another (thanks Delta) with a 40 minute layover! Jet Blue wasn't bad either, we had to fly out of Boston, rather than Providence to get it, but couldn't beat the price ($69 one way).
It's so much better to fly in the day before. At least that way, if you're delayed, you won't worry about missing the cruise.