Or maybe they are breaking free BECAUSE the sea level is rising.
Ned answered the part about "already afloat" ice sheets - the big concern seems to be whether the partially grounded ice sheets and the humungous glaciers that feed them are accelerating their journey back into seawater, possibly accelerated by rising sea levels. There's a lot of research activity on the subject, but I'm not knowledgeable enough in it all to evaluate any risks.
Though covering half of Florida might not be all
that bad.