Seems all too harsh! The good news is that I managed to reproduce it.
I tied one dog to a tree at open mire, with open water next to it. I observed the dog wading in there normally, with no ill effects.
Then zoomed-out, waited for some time, and returned - and the dog had drowned. So it seems the idle-time checks, when the player is not on the map,
are to blame for the creature drowning. Now this is likely a same mechanic that makes the NPCs still sometimes to drown in village ponds.
Chances to fix this for both the dogs and NPCs are now very good.
Sorry for your loss, thanks for the report.