Watermarks are a very good idea. You don't want your pictures being used by other people for scam.. (like to sell their product) And you definitly don't want your pictures used for any scam.
Watermarks are lovely. It is the artists signature on your piece, it should be nothing to be ashamed of.. Watermarks that are too big are just obnoxious and ruin the original piece. Also, the watermark should be a way for other people to find your photo's. (like someone already stated)
Honestly, I don't think there is anyway to avoid people taking photos without having an obnoxious watermark, but having a watermark shouldn't be as important as not ruining your photo.
Anime Expo 2014
Anime California 2014