Check the buoys, they won't lie as they are sitting on the water. 48 in myrtle, 54 at wrightsville. 1)Myrtle--- http://www.ndbc.noaa.gov/station_page.php?station=mros1 2)Wrightsville---http://www.ndbc.noaa.gov/station_page.php?station=41110
The water temperature for Myrtle Beach on Swellinfo, is taken from the Spring Maid Pier buoy, which is about as good as you are going to get.
some of the differences have to do with wind.. stiff offshores can cause an "upwelling" effect, dropping the temp up to 10 degrees within hours. but yeah, it aint 66 degrees yet...i wish!