It will definitely be warm enough to swim in October, and most likely early to mid-November, too. I live in NW Florida and it's usually still pretty warm here in Oct, sometimes even hot, although it usually cools down nicely at night.
I was at WDW a few years ago in early November for a conference and it was quite humid and rained hard for 2-3 days, although I think that is unusual. Oct-early Nov are usually fairly dry.
Our favorite time to visit WDW used to be early Dec, right after Thanksgiving holiday, but the secret is out and it has become more crowded over the years. The weather is just about perfect, though. Now we go in mid-January (AFTER the WDW marathon) and the crowds are very low, the resort prices are discounted, and the weather is, well, shall we say interesting. It can be very cold (as in frosty) in the morning, sunny during the day, cold again at night. Or it can be very mild. I've never tried swimming in the pools in Jan, even if they are heated, because eventually you have to get OUT of them!
Anyway, Oct-Nov is a great time to go, especially if you can avoid those holiday weekends like Presidents Day and Veterans Day. Have fun planning!
Bev