We've been there in late January and it was too chilly to swim in the ocean.
Air temp was low to mid 70's.
Early December you should be fine, barring a freak cold snap.
Just watch the weather in Orlando/Port Canaveral the week you leave.\
Nassau is straight out from there, so the weather fronts move off the Florida coast and out to them.
So if Florida is in the 80's around your cruise, you will be fine.
Legend - Sept. 18, 2011 - Cozumel, Belize, Roatan, Grand Cayman
Fascination - Half Monn Cay, Nassau
Nov. 13, 2010 -
Sept. 18, 2010 -
Sept. 19, 2009 -
Inspiration - Jan. 10, 2010- Grand Cayman, Cozumel
Freedom - February 8th, 2009 - Key West, Grand Cayman, Ocho Rios Jamaica
Glory - September 8th, 2007 - Key West, Cozumel, Progresso, Nassau
Sensation - Freepor-Nassau
Jan. 28th, 2007 -
Sept. 10th, 2006 -