It's better to take the postgraduate entrance examination in mechanics of materials, but if you want to enter the postgraduate entrance examination in water, you still have to look at hydraulics or fluid mechanics after the second interview. If there is a big difference between your major in undergraduate course and your major in postgraduate entrance examination, you should take two more courses with the same academic ability, one is physics, and the other is other courses that you didn't take in postgraduate entrance examination. Choose one, and you will be admitted to hydraulics.