The Only Water in the Forest is the Complete Failure to Set That Up Right.
Stephen Moffat, did you or did you not just have a character show up at the end of your "OMG so amazing!" episode to explain the plot to us because you had just failed utterly to do so?
And no, "Well, it's a children's show" does not excuse that.
I have other thoughts, but this is the one I keep coming back to.