I have heard of others being able to do this. Was wondering if this is a bug or intentional feature.
For those who don't know, if you are at water level, you can drop dirt on the tile, then have multiple people attempt to remove the dirt. The first person who activates the activity removes the dirt, but everyone else will continue to remove material, thus lowering the tile further and deeper into the water.
I haven't tried it myself, so cannot provide firm details. It's a pity there isn't a way to fill up a ditch with water though where players could build a proper moat.
Then again, if I could build a moat, I'd want to build a draw bridge as well.