You can submit anything you wish to your CGPortfolio. If you’re submitting to CGTalk’s 2D or 3D gallery then your work will be more carefully scrutinized for quality and content. The derivative work guideline is referring to work that closely resembles the work of someone else. Not commercial work that you have done.
Watermarks are a tough call. You obviously want to protect your work, but if the mark obscures too much or is a distraction to the work, then it’ll probably be rejected. It’s hard to say how much is too much. Keep it tasteful and as minimal as possible, is the only advice I can give.
Of the two examples that you’ve shown and going by recent approval/rejection history of CGTalk’s galleries I would recommend posting to the WIP forums first for advice on how to improve these images before posting to the galleries.
My critique of the images: I’ve seen many images rejected for mono-color backgrounds and static 3/4 view compositions. Your lighting on the tank is also confusing. It seems to have rim highlights on the top and bottom.