Opps moment on my part but I think bug in the end. I had them building a platform and digging above some water when they dug themselves on to a ledge which I made in red. I had JUST run out of materials so iI could not build a ladder and they stood there suffocating. I then just dug underneath them and they fell down into the water and stood there and drown. They would not leave the water. Either way, getting stuck without materials to make a ladder might be a real thing.
Run out of resources when 3 or more squares above water