We have camped at FW in Mar, May, August & November.
mid March 07- weather was great high 50's at night, high 70's low 80's during the day. No humidity. Rained once the entire week. However, the week after we were there, they had freezing temperatures in Orlando area. Parks were busy. Swam a bit but it was cold when you got out of the pool.
Early May 2004- high 50's at night, low 80's during the day. Low humidity. Best weather ever (no rain at all). Crowds at parks very light. Used the pool twice during visit.
Early August- Sweltering hell-box of despair. Extremely hot, humid (and how). Rained every day. Parks were very crowded. Swam continuously. Woulda stayed in the pool the entire time if I could have arranged beer delivery service.
Mid to late Nov.- low 50's at night. Low 70's during day. Quite a bit of rain. No humidity. Parks were empty first 4 days and then swelled to unmanageable size for T-giving weekend. Pool? No way. Hot tub, maybe.
Florida weather (like Texas weather in general), is notoriously fickle and can change "on the fly". Best advice when traveling in Spring is to come prepared for warm and cold weather. For me, this generally means throwing a sweater, hat and a light jacket into the suitcase along with my normal clothes.