To give another deviant a direct link to this FAQ answer, use the following code in your comment or forum post:
Does deviantART have a feature that will automatically watermark my deviations?
Yes. This option will be presented to you only if you change the Display Options field to one of the smaller resolutions available in the submission/editing process, allowing you to add a watermark or not.
Images which are kept at 'original size' will not be processed by us and will not present the option of automatic watermarking on your images.