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).
Now if you accepted some Terms of use on a website that displays your pictures, you want to read them in detail before you react (perhaps the TOS says that by posting your pictures on their site you agree to this and that).