Copyright rules apply to all your works, watermark or not. An absence of explicit licensing conditions does not mean that you grant rights of use to anybody, on the very contrary. If you can prove you are the author (which should be easy enough in your case), and if they can't prove they have a licensing agreement with you that gives them the right to do whatever they are doing with your works, then they are in trouble (whether they know it's yours or not).
Does everyone at the charitable organisation and the agency in question work for nothing? That is, are they all volunteers?
I am all for helping charitable causes, and indeed if they ask usually they'll be told they can use something for no charge, but sometimes in situations like this I get the feeling "charity" becomes an escape clause for behaving irresponsibly or hiding behind not having to pay fair rates.