The Way Way Back
(2013)

Trent (Steve Carrell) is having an affair with Joan (Amanada Peet). Duncan (Liam James) spends most of his time at the water park headed by Owen (Sam Rockwell). Before Duncan leaves with his mother  Pam(Toni Collette) and Trent he goes to the water park and succesfully passes Owen in a water tube.

In the end it is implied that Pam will leave Trent.

Thanks Michael R!