circle lake using radius
desired output(canva edited)

I want to know how can I use the real shape of our local lake which is our study area surrounded by land to get the desired look of the 2nd image.

the first lake patch was created by creating a 25px radius from the center of the environment that would serve as the lake. One problem is that the agents in the lake can come out of the lake area which shoudl’nt happen. Now, I want to replicate the shape of our study area lake. Im a complete beginner. I tried GIS extensions and polygons but Im having trouble with it. I really need some guidance from an expert right now. I appreciate any insights.