We have been in March 5 times in the past (sometimes end of March sometimes the start.) This year we will be there the 1st week. I find it has more to do with the year then the month you go. This year seems cold so far, I am kinda betting it will be on the chilly side (70's) and plan to have more jeans and sweat shirts then in the past. However some days can be great 80's, and in the same week 65. All depends.
In thoses 5 years we only had one that we remember as being "cold". 1998, an Elnio year, same as this year. My hubby only packed one sweat shrit, we came home with a new one from WDW.
Don't fear, it could warm up and all the pools at WDW are heated.
Lets both pray for warm. If not as long as you are dressed well it is still great fun. (and almost always better there then it is at our home in Michigan!!!)