when we have gone in mid-march in the past temps have always been in the mid-80's - my kids were really little so we weren't at the swimming stage - but the pools were always full
Last March we went. In fact the last week of March and it was chilly. 50s 60s. You can never tell. Do a search for orlando news stations and keep checking. another is www.accuweather.com for a 15 day forecast.
allears has a historical weather page and it says avg temp for March is 67 but there is also a record high of 91 so you never know. I'd pack a swimsuit just in case (they don't take up much space, LOL!!!!).