This is a view from below the ocean surface of a female humpback whale (Megaptera novaeangliae australis) turned upside-down, slapping the water with her peduncle and fluke, i.e., rear end.
Quite the sight to behold, and the percussion to hear/ feel.
There are probably many reasons why humpbacks do this. In this instance, she was engaged in activity with her calf, who was off breaching, slapping and practicing other important baby-whale stuff.