Well, they sure look like mites to me, added to the fact that the snakes are soaking....
At this time of year mites are slow to breed, so mite populations remain low. Adult snake mites are about 1/4mm long, visible to the naked eye, but only just. I use the 'pop' test; any thing found floating in the water is popped between my fingernails. If it 'pops' - it's a mite.