Sister Swimming

There is a giant pool being fed by a waterfall. It spreads out into the horizon so far that I can't tell how big it is - but huge. There is some sort of game going on here. I am paired with my sister and the goal is to move into the pool, step by step, gradually going more underwater. First just the feet, then the legs, but when I can no longer touch the ground I pick up my sister and sort of swim-carry her into the deeper water. She gets angry at me and swims away off to the horizon. Later, I am back on the beach-area and my mother tells me that my sister is mad at me because she thinks that I think she can not swim.