The reason I watermark photos is mostly so that people viewing the photo can easily see who took it and where to see more (since the net is great for distributing things far and wide). If a cosplayer edits a photo, even to cropping the watermark out, as long as they still credit me for taking the photo I have no real issue with it. Heck, I'm really not great at photoshop, so I love to see what people do.
That said, it's always a good idea to check if you're planning to do something massive, just in case. I'm easygoing, but some photogs have a specific intent they don't want lost for their images.