As I said, far be it from me to contradict your instructor, but if Tmx 400 is what you are familiar with, it is what you have "dialed down" and you know how to control, why mess with something else? Specially in a workshop situation where you want the best negatives. As I understand it you are going to a pt/pd workshop, diffraction is not an issue with contact printing.
If you want the fluffy looking moving water just close down more, or use a polarizer. I really think you should give Les's trick a try, I think you will be pleseantly rewarded.