March weather can be unpredictable.
We went 13th/27th last year and we had some lovely weather, some days where it rained all day and others where it was on the cool side (especially in the morning's).
Our day at Discovery Cove was ruined by a 2 hour thunderstorm that delayed our Dolphin swim, plus the water was freezing cold.
Being from the UK we are now tied to the August school holidays and whilst it is very hot, I would much prefer to go then.
Last year's temperatures were lower than average though, there is a good chance this March the weather will be good, I do not think you can guarantee it though.