I'm pretty sure others will have mentioned this as an issue, but adding in something that just happened as a new data point or two. There's an underwater cave in the SW area of Ocean biome in genesis. We've had a couple of tames instantly die there, as well as one player death and one sleeping player death, all instant.
What is a new data point though, I think, is that just now I was trying to tranquilize an X-mosa in the pool that's in the middle of the cave. I was standing on shore, it was in water. I was monitoring it's torpor using HLNA info, so it was at around 9-10k of 12k torpor with almost full health when it instantly died. So it wasn't a tame, wasn't unconscious, wasn't massively damaged. This happened on genesis 589 at about 8:15pm PST. I have a couple of screenshots of the mosa if it's useful, but obviously it won't appear in my tribe log. I can also hunt down the relevant other issues we've had with tames dying in the log if that's useful but I'm sure others have already reported.