First off, it's clearly a stage on which some sand has been spread. The painted backdrop of a pier jutting into the water is quite apparent.
What also seems obvious to me is the sexual content of this short. The man wears a top and bathing trunks, but he clearly is wearing a codpiece as he plays stickball with the two pretty young women in sailor suits, then holds on for the other's spanking.
Of course, it's possible that I'm a dirty old man who sees filth in even the most innocent activity. To the pure, all things are pure, and to the low-minded, everything is impure. I'm sure that's what they said to the local bluestockings who objected to this.