Grrr... Let me try to explain it better. When you vibrate the water at 24Hz, the water will go throw a cycle that has 24 distinct positions - so you could take 24 pictures where the water appears in a different pattern, but after that it will go back to pattern #1.
Now, the camera is taking 24 pictures a second, so if the water is vibrating at 24Hz, each frame of video corresponds to one of the 24 positions created by the 24Hz wave, which makes it look frozen in air.
If the water is vibrating at 25Hz, it has 25 distinct positions, so the camera takes 24 pictures in the first second, and then the first picture of the second second is position 25, so then next time around the first picture of the third second is position 24, the first picture of the forth second is position 23, and so on. So that makes the water seem to inch forward at one frame (position) per second.
23Hz is the opposite - the camera takes 24 pictures in the first second (which includes the 23 positions plus position 1 of the new cycle), and the first picture of the second second is position 2. Next time around it is position 3, and after that 4, 5, etc. So that disparity in the other direction makes the water seem to inch backwards at one frame (position) per second.
Sorta like how when you see footage of car wheels sometimes they appear to spin backwards. That happens when the frequency of the spin gets to be near the frame rate of the camera or a fraction of it (like one half but slightly less than that).