Florida Ocean water in February is about as hot as the water ever gets in Massachusetts. It NEVER gets as warm in California water, which is affected by the Alaska current, and requires that you wear a wetsuit for an extended swim.
Yeah, you can get hypothermia even in Florida in the winter time, if you stay in too long. Remember those folks who crashed a plane off Cat Cay a few years ago, and were in the water a couple days. Not all of them survived, even though the crash landing did not hurt them.