I have not been to Disney World during March, but from things I have read and heard, it seems the temperature is usually in the upper 60's to low 70's, with maybe a day or two slightly colder (upper 50's). Also, there is only a fractional amount of rain in March compared to the summer months. You will almost assuredly see a some days of swimming weather....even visitors in December or January have been known to find a couple days a week where swimming is possible. Plus all hotel pools are heated. Even on cooler days you can definitely swim, its just the trip back to the hotel room that can be the killer!