I just got back to playing df after a brief pause and while in a couple of bomb the carrier missions I observed the enemy tanks floating and firing at the blue carrier. My question is when did tanks become aquatic. Is there different types of tanks or are they all floatable?

Sometimes the server doesn't match properly, so they look like they are on the water, but to them they may be on the ground. Just like sometimes you may see a plane or tank go underground.