We go to HHI every year for the week between Xmas and New Year's (we also go in the summer) - it's my favorite time of year there. Biking and tennis are the best, then. So much nicer when it's cool. Also better for sightseeing in Savannah. We often see mid-60's and an occasional 70 degree day. To me, that's perfect weather. It does get cool to cold at night, even down to 40's sometimes. I usually layer with heavy sweatshirt and lightweight jacket - never bring a heavy coat, not necessary. Like sernow said, golfing is great that time of year.
If you DO golf, see if there's any availability at Heritage Club. It's not OF but in a very nice location in Sea Pines, has a heated pool, AND includes a GREAT golf package. For swimming in a heated (outdoor) pool in off-season, I like the pool at Surfwatch best, Barony second. They also both have indoor pools.
BTW, I do lay out on the beach sometimes that time of year. Too cold to swim in ocean but warm enough to sometimes lay out in swimsuit when sun is out. Great to walk on beach also.