When something plunges into water, the eventual dynamics of the water is immensely complicated. You, of course, are not supposed to analyse those processes that are too complex or even impossible to analyse (for instance, when instabilities develop, the dynamics become chaotic, i.e. unpredictable). Instead, try finding something that is easier to analyse.
This cannot be considered as a real hint, so there will be no reduction of the base score.
Please submit the solution to this problem via e-mail to physcs.cup@gmail.com. There Hint No 1 will be published on 5th November, 13:00 GMT.