Anytime you publish any work -- online, print, video, dvd, etc. -- there is always the chance that someone can copy it and steal it. That is simply the way things are. Watermarking your renders is a good way to deter potential pirates, but, realistically, if people want to steal it, they'll find a way. If you don't want it stolen, don't publish it.

Copyright policies are really sticky and are different depending on the school you are attending. As suggested by dbirider, I would highly recommend talking to a legal counsel at your school to get the correct information. Your instructor may believe one things, but the reality of the situation could be very different.

It is my understanding that the university, faculty, and department can do what they want with your work, but they cannot claim credit for it. Also, they cannot sell it without a) your permission and b) sharing the profits with you. However, I could be wrong on this. Again, check with legal counsel to be sure.