I was swimming (didn't have a choice but to get it, I started on a tiny island with four other people spawning at the same time, barely enough lp resource to get swimming and get off) starting at full stam. I was crossing a stream with a couple of tiles only of deep water. I got right up to the opposite shallows with maybe a bit over half stamina. Then I drowned.
There was a bit of a lagspike at the time (or at least the character twitched a bit while swimming, so I assume it was a lagspike), so maybe that was it. Or maybe the stamina bar doesn't display swimming weariness correctly? Or, hell, maybe you die if you get down to half stam while swimming.
Either way it sucked