So I caught my Hearthlings doing something a little… strange.
What I can only imagine they’re saying is;
Blurbblyy blurb blurb blub blurbblyy!
Blurb burbly burb blurb blurb!
Which loosely translates to;
Hey, we’re underwater!
We should probably go get some air!
Wish this was somehow taken care of, I had a game where I walled myself near river, thinking how I can’t be atacked, sadly I had to break my perfect defense because my herbalist kept teleporting into water and then admiring a tree. 
This just made me laugh so hard. 
Well, being underwater actually gives a buff (although an invisible one for the player) which could easily be edited to add a fast damage over time to represent drowning.
That alone would become a mass murder machine, however. If you do that, you would also have to tweak AI to make them avoid water as much as possible (not idle in it, chat in it, etc) and perhaps even somewhat “calculate” how long their trajectories under water will be so they can decide not to go if too long; which is… definitely far more complicated.
But doable, for sure.
It would be nice to see water become more of an environmental weapon in terms of defending a village, right now I’m digging moats around my village to funnel incoming attacks along paths I desire, with high walls, a long bridge and some well-placed archers, attacks are virtually nullified.
That being said by the time I’m at the level to build walls to repel invasions the enemies aren’t exactly tough to defeat on a level playing field. Mostly I do it for the aesthetics anyway but if someday in the future larger raids with tougher enemies were to come in then I’d love to test my defences against something truly dangerous.
It’s been said before many times over many posts I’m sure but the Engineer could be beefed out a little more, trap doors, pit spikes, even the most simple of defensive traps can pack a huge punch when deployed right. If water was to be given some sort of drowning effects as you mentioned @DaniAngione then things like moats, rivers and lakes would offer much more than just scenic beauty and a natural obstacle as they do now.
A man can dream, a man can dream.
This would be just awesome
That was what I was planning at first. I’m using mod that generates rivers which, at least in Arctic give awesome possibilities to use walls in early game. 