R. L. Stine’s The Haunting Hour Season 1 Episode 18

Kai must overcome his fear of the water in order to convince people there is a shark in the pool. It is suspected that the shark is actually a Nanaue (a creature that is a shark in the water and a man on land). It turns out that the shark Kai was seeing was actually his father and that alongside his dad, he is also a Nanaue. Kai uses this as an advantage, scaring the lifeguard that bullies him and taking his crush for a ride in the pool on his back in shark form.