Not only villagers die in shallow water. Robbers can too - for example they enter water if PC is on the boat shooting at them. They won't move to the deep water, but are "running" only in shallow water, sometimes even getting out and walking away for a while. They are tiring when in water, and if they tire too much, they drown. The log entry is for example "The Vagabond robber seems to have drowned".
Interestingly, they don't drown (usually?) while lying unconscious on the water, they almost always regain consciousness only to drown a moment later.