Forget about the swamp ones there's never anything in them, usually drauger. I wonder if it's a reference to "evil dead army of darkness

Failed boss spawn point.

There's no swimming but you could build a diving board and dive to the bottom. Useful if you drop something in deepish water and need to get close enough for auto-pickup. In this particular scenario you might even manage to get under a staircase and stay underwater.

You can trap abominations in it too :p
But if it's flooded, you can't get to the bottom.

It's decoration mostly. Draugr might spawn near it. They have them in the mountains too without the water.

the ones in the mountains can have chests in em tho; i had to tunnel thru a rock to get to it, but it was there.