The gravitational pull of the moon.
See, if the moon is facing a certain side of the Earth, the ocean that the moon is facing(and the one on the complete opposite side) will affect the water tides, calling it a High Tide.
For example, if the moon is near the East side of the Earth, the ocean on the East AND West will be high tides. The North and South oceans will be Low Tides. And vise versa. (That example was supposed to be like a diagram)
(I tried. Sorry)
