To begin we firstly need to see what a watermark is. A watermark is a text or an image that is on what you want add on your photo or video to enter under the copyright rules. Basically, anything that includes a watermark is prohibited to be reproduced without written permission of the author in law.
Types of watermarks
Watermark with 50% opacity
A very inefficient method although it is one of the most used watermarks: white text with 50% opacity.
Watermark Color and Opacity 50%
Adding color to the watermark makes the removal to become more difficult and if someone manages the color removal will see more than some contours which can be interpreted as an image compression distortion.
Watermark Overlay + color + 50% opacity
A widespread myth is that the watermark can not be removed by changing the blending (blending mode). Some "blending" modes such as Difference will produce results that can not be removed but the most common watermark is the Overlay blending that can be easily removed. Overlay mode removal is difficult and will still remain traces of watermarks, but often the text deleted is not intelligible.
Emboss + 50% opacity
Adding a filter such as Emboss can greatly hamper the watermark's removal.
Someone may remove a watermark effect , but he must know exactly how to replicate the effect and then to find out how to remove the watermark. Even if the person is able to duplicate the effect 100% accurate, on the final image might be more likely to still read the initial watermark. The embossed watermark is one of the most hardest effects to remove.
How to create safe watermarks
Place the watermark on a charged background. This method prevents you from using very simple techniques.
Use a high watermark. Use a watermark that fills almost the entire image and thus can not be removed.Clone Stamp and Brush will be useless at the watermark removal attempt.
Use opacity at least 50%. As opacity is lower it is easier to remove the effect and the rest of the picture/video might be very less visible.
Make the watermark difficult to duplicate. Some methods require advanced skills even up to 100% duplication of the watermark. To make the watermark hard to duplicate use original fonts, types and effects settings. You can add besides the text a logo or a unique other form.