Hi and welcome to the DIS!
EE does not yet have an OFFICIAL opening date. Rumors have it opening on April 22, with previews (soft opening) starting in February or March. But that is pure speculation.
Water is heated in the WDW pools and water parks. The air can be cool though!
I don't know what part of March you plan to go, but early March averages are in the mid 70s during the day and mid 50s at night. You could get some days in the 80s but you could also get some mornings in the upper 40s.
Late March averages are low 80s in the afternoon, upper 50s in the monring. Mornings in the 40s are rare, but still possible.
Bob