Coastal Water Temperature (Fahrenheit) for year 1999 Month Myrtle Beach, SC Key West, FL January 48.0 69.0 February 50.0 70.0 March 55.0 75.0 April 64.0 79.0 May 72.5 82.5 June 78.5 86.0 July 82.0 87.0 August 82.5 87.0 September 80.0 86.0 October 69.5 81.5 November 61.0 76.0 December 53.0 72.0