As you said in March it can still be a little too cold, but I've been watching the weather for all of March and it's been in the high 70's low 80's all month.
We typically go in mid-April after spring break, this year we'll be going April 17th to the 22nd and the weather is usually awesome, low 80's not humid at all.
We went last year in the middle of May and it was in the high 80's and more humid, beautiful weather like in April, but a little too warm for my blood and it was busier.
Remember most of spring breakers are going to Orlando between now and mid April from around the country, 3rd weekend of April is great.