Technically, under copyright law, once you state that ALL content on your website is a registered copyright, then a blanket of protectionj essentially envelopes the entire site from unlawful use. Also, your images will be small and impossible for anyone to re-use without serious degradation to the image. However, there is nothing to stop someone from downloading your image and sticking it on their website. But, you have the weigh the pros and cons and what you want to say with the pics. As a professional photographer, I don't like those watermarks on my images as I find them quite distracting from the image. So, I opted to notify the viewer that my work is copyrighted, so bugger off!
But, if you want a watermark, the best is to do a batch action in PS. You make it once, record it, throw the images in a folder, and away you go.