Gravity Falls (2012)

S1E15 The Deep End

Mabel encounters an adorable merman stranded in the local public pool and is resolute in her mission to rescue him. At the same time, Dipper seizes the opportunity to become a lifeguard, hoping to impress Wendy. As Mabel devises a plan to free the merman, Dipper navigates the challenges of his new role, leading to a summer full of unexpected adventures and aquatic antics.