The Bug:

Foxes sit by themselves every once and a while as an idle animation. While swimming, you would think they wouldn't attempt to since that doesn't make much sense, but it seems that nothing is stopping them from doing so.

Every once and a while while swimming, foxes will attempt to swim, sink a little bit, then instantly get back up after their head is submerged. 

Screenshot:

Sorry for the weird screenshot, this actually happens fairly quickly so it was hard to get a good one.