The Lake is one of several visitable locations in Adventure High.
At present, only a few scenes acts out here.
The first date with Sarah takes place here. Telling her that her favorite color is blue and asking her about the lake unlocks another scene here later, after which Sarah unlocks the Bubble spell if you decide to tag along with her.
If you have Sarah and Cassandra in your party and watch all four scenes in the Dungeon, then talk to Molly, a scene here takes place where Cassandra considers changing gender simply to be able to date Sarah. She ends up deciding against it, throwing the sex change potion into the lake.
If you used Flirt on Julia, she can be found here. You can also go here on your date with her.