Aqualorius! 2009 on netflix

20 years ago two boys disappeared in the swimming pool at Hulgaard Elementary. Ever since children have been telling stories about a monster living at the bottom of the pool. A victim of all the school bullies, 11-year-old Lars, has written a love letter for the older girl Sherin. When Sherin’s boyfriend, the worst bad guy in school, finds the letter, he forces Lars to swim to the bottom of the pool to retrieve a shining coin. But Lars is convinced that the monster Aqualorius is waiting in the deep…