The Thirteenth Year is a 1999 Disney Channel Original Movie. It was released May 15, 1999 and is a mix of comedy and fantasy. The film was directed by Duwayne Dunham, who has directed other children and Disney movies, including Little Giants and Halloweentown. The ocean scenes were primary shot in the Newport Beach communities of Balboa Peninsula and Corona del Mar.
Cody Griffin is an adopted teenager; his birth mother is a mermaid who left him on the Griffins' boat when he was a little baby to avoid being captured by Mr. Wheatly, a fisherman who later became obsessed with finding mermaids. Cody establishes himself as a quick swimmer in his town, and has a girlfriend named Sam. He's under a lot of pressure from a big swimming tournament coming up and he's also failing biology. He later saves Jess Wheatly, a geek, from drowning. Later they become buddies. After Cody's 13th birthday, he begins to feel strange.
As he wakes up in the morning, he turns off his alarm and zaps it. Ignoring it, Cody drinks milk from a carton and it gets stuck to his hand. Later he starts drinking a lot of water, claiming he's always thirsty. He goes to Jesse for help, who agrees to figure out whats wrong with him as long as Cody teaches him to swim in return. Jess runs various test learning Cody can talk to fish, swim extremely fast, climbs walls, generate electricity, and when sometimes wet, scales appear on his arms and hands. Jess concludes that Cody is transforming into a merman.
Despite all the things happening to him, as well as the objection of his adoptive parents when they found out, Cody still attends the swimming tournament. He not only wins and beats his teammate slash rival, Sean, buts he breaks the state swim record and his scales appear on his arm. Sean sees the scales and thinks Cody cheated, following him into the locker room. Cody ditches him by hanging on the ceiling. Cody safely makes it out of the stadium and makes his way home with his parents after they find out he wasn't at home; but now his scales are lasting longer and even appear on his legs.
Cody tries calming his mom and dad by telling them that nobody saw him change, but Jess shows up and tells them that he saw everything and people are talking about Cody. Sam arrives and Cody explains to her what happened. She doesn't believe him so when he shows her his scales she faints. When she wakes up she is freaked out. Cody tells her he is still the same guy and not to be afraid. While promised to secrecy for Cody to be safe, Sam, shocked by the discovery, leaves. The next morning Cody takes a swim, and meets his mermaid mother, however, Jess' father spots them and ends the reunion.
Later that day, Cody meets Sam at the beach. She apologizes and then they share first kiss. Cody walks into the water with her, saying there's something he wants to show her. Cody's Birth mom then appears, with Sam complimenting her beauty; however, Cody suddenly gasps in pain and stumbles back to shore. Writhing on the ground, he tells Sam to get his parents. After she left, Cody's feet turns in to flippers/fins. Thinking Sam was back, Cody is surprised when someone throws a blanket over him.
Jess comes down to where Cody is, but sees he is gone. Jess sees his dad's boat and panics because his father had seen Cody's fins earlier. Jess's dad is planning on using Cody to lure in his mom to prove he's not crazy and mermaids exist. Jess comes on boards the ship, and asks him for help. Despite trying to warn the mermaid away, Cody and Jess are unable to stop a fishing net from trapping her. Cody asks Jess to help his mom, so he takes a knife and jumps into the water to cut the net. He is successful, but his legs get caught in the net, pulling him under. Jess passes out, but Cody dives in and brings him up to the dock where everyone is. Sam performs CPR, but is unsuccessful. Cody uses his electricity power to zap Jess back to life. Jess then wonder if Cody's girlfriend was kissing him, causing everyone to laugh. Cody's mermaid mom shows up again. Cody explains she wants him to go with her because she's the only one who can help him with his changes. Cody adoptive mom doesn't want him to go, but his birth mom promises to bring him back before school starts. After they all say their goodbyes, Jess asks Cody to tell him and his dad about the aquatic life he sees after he returns, and Sam makes Cody promise not to fall in love with mergirls. Cody dives into the water and finishes his merman transformation with his mom's help. He swims back up and leaps through the surface, and waves goodbye to his friends and adoptive parents. Cody and his mother swim off into a light below the surface.