My family and I have been to WDW in March a number of times, both before, during and after Easter. We have always found the weather to be very pleasant to very hot, and have spent a great deal of time in the pools.
That being said, we're coming from a climate where the average daily high temperatures in March are still in the high 30's, and it's 50/50 whether it's still snowing, so anything over 65 is definitely shorts, t-shirt and swimming pool time for us, and anything over 70 means it's time to turn on the air conditioning.