We usually go in March to escape the winter blahs (unfortunately winter is still here when we get back). It is pretty crowded though not at its worst. We use fastpass a lot. For example, the wait time for Test Track can go up to 90 min to 2 hrs though it is not always that long. Standard wait time for Dumbo is 30 - 45 min (why oh why can't they put this one on fastpass?). We have found though that if one park is particularly crowded (namely MK) then the others are not nearly so bad. Also, AK and MGM crowds always seem to be pretty reasonable.
The weather has always been pretty warm to hot though you might have some days (or just the morning) when you would be best to wear a sweatshirt and long pants. Some rain is a possibility but not necessarily a probability. I guess the later in March you go, the more likely you are to have warmer weather, however, having been at all times throughout the month of March, I can tell you that there have been times at the end of March that were way cooler than times we were there at the beginning of March. We have always been able to go swimming at least some days and we are not polar bear types.
HTH
