The water dynamics become extremely complicated very fast. Basically, the only thing that can be reliably measured and used (based on a few frames before and a couple of frames after the impact) is the instantaneous change in the speed of the cylinder during the impact.
Please submit the solution to this problem via e-mail to physcs.cup@gmail.com. The Hint No 2 will be published on 12th November, 13:00 GMT.