if you want no watermark you have to pay.
the good news is, there's a student version available for about $400 (currently also includes free upgrade to Maya 8).
As far as i know, the student license is exactly the same as the production license that you buy for ~6000. But you cannot use the student license for commercial work, unless you want to pay a massive fine.
The student version has the functionality of Maya Unlimited.
You can find it at http://www.studica.com/alias