The last 2 years we've visited in mid-March. Neither was warm enough for the ocean, but the pools are heated, so we got to swim both times. 2 years ago it was in the upper 70's every day. Everything was in bloom, and it was great to be walking around in shorts in March.
Last year, all I can say is BRRR!!! I think it stayed in the 50's during the day, and dropped into the 30/40's at night. We did some sightseeing in Beaufort, and just enjoyed spending time together in the evenings. My husband had brought his laptop, and I brought my scrapbooking supplies. It was real nice to enjoy our hobbies without worrying about the usual daily stuff we all deal with. Even in March there were activities at the resort every day---good mix of things for kids and adults.
Regardless, all of the restaurants were interested in were open both times. HH seems to get a ton of snowbirds who winter there, so restaurants stay busy, even in the winter.
One last thought, even though we didn't swim in the ocean, it was nice just to jog or bike to it everyday. We flew our kite on the sand, and that was fun too.