In mid-March, you will likely have nice days that aren't all that humid. (Then again, I live in Florida, so less humid weather to me will probably feel humid to someone from a dry climate.) I have gone to the water parks during this time. You might have days that might be too cool, but if you go in the afternoon, the temperature will be warmer. When packing, I'd suggest items for layering. Then again, what I might consider to be "cool" (as a Floridian), you might think is warm weather!