Not sure about Myrtle's Oceanwatch, which I suspect has lower occupancy rates than Hilton Head in the off-season, but Hilton Head's Grande Ocean is always above 95% (and many times 100% occupancy) in the early months of January, February, March, and then at 100% on through November, with December being slightly lower.
In recent years we have loved going to HH in the "off-season", and enjoy it equally if not more.......it's different but great. We now stay at GO in January, February, March, and April (and again in June), and all of those times are very busy. There are several TUG members who visit HH (and GO in particular) in Dec-Jan-Feb for 10-12 consecutive weeks. They probably consider the
summer to be the off-season, and I wouldn't argue with them.
To say HH is strictly a summer destination is to miss some of the best times to be there on the island. During most Jan and Feb months we see several random days in the 60s, and they're ideal for golf, tennis, walking, and biking.....not to mention shopping and wining & dining. My wife participates in the GO tennis clinics every single day (weekdays) in Jan and Feb, and the class is full with about 12-14 participants, and has two full-time tennis pros.
We've met several great couples who come each year and have become very close to them---(stayed with one couple last September in Jersey to attend the US Open Tennis Tournament in NYC.)
It's true that Spring Break times can be iffy, but that's always the case during the fringe months, but the corollary to that is that many times it's warm in any of the early months too! The absolute best month imho is April, when it is more often than not absolutely ideal. Those cold times are the exception.
Anecdotally, the longest wait we've ever experienced at Skull Creek Boathouse restaurant occurred on a Monday night in January, and the wait was 2 full hours. (That night we decided to go spontaneously and didn't use the concierge call-ahead perk, so we had to wait.)
.