We've been inside, around, and on top of a Sherman tank. Despite the fact that plenty of tanks before and after were heavier, the Sherman's still an ominous beast. So how to make one float? Give it a big collar filled with air, seal the lower hull and stick a prop on the back. Design to provide amphibious infantry support to invasion forces, the DD's didn't do well in rough seas, and they fared especially poorly at Omaha Beach. Still, the fact that they actually made the damn thing work is pretty impressive in and of itself.
DD Tanks [Wikipedia]
Gibbs Humdinga, Big Brother to Aquada Amphibious Vehicle [Internal]