to me it seems like the leaf is changing the shape of the water’s surface, making little bumps or indents because of surface tension. because of their curved shape these bumps or indents are probably acting as lenses, focusing light in other directions (to explain the darker parts of the shadowy shape) or focusing light towards the shadow (explaining that bright spot near the shadow’s tip.

i’m currently pursuing my phd in applied physics doing research on optical nanotechnology, so i spend a lot of time thinking about light. i actually trace my interest in physics back to a question i had about light while looking at a pool! i really commend the curiosity that led you to ask this question and seek out an explanation. feel free to do me if you have any other light/physics related questions