Seems they are always a new drowning wound, I have had 15+ on my torso at once and they never stacked.
Also, it actually seemed like this heal slower to me. (not actually measured though, just a hunch)
AvatarJTC:
I lead a tame dog while on a boat from Kinsaitos to a kilometer east of Samling.
That journey was all on water, and the dog swam the entire time. It is safe to say dogs are not affected by drowning.
Further testing with bears and crocodiles will be required before we are certain animals don't drown.