Does deviantART have a feature that will automatically watermark my deviations?

Yes. This option will be presented to you when using the resizing tool in the submission process, allowing you to add a watermark or not.

To place a watermark on older deviations you will need to edit the deviation and re-upload your file, only then will you be presented with the option to watermark your piece.

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.