When a mission between the Teen Titans and the X-Men put Starfire, Kitty Pryde, Raven, Rogue, Jinx and Jean Grey into mermaids, they discover that an evil Leviathan wants the Pearl.
- Teen Titans:
- Starfire (Hynden Walch) - her mermaid form consists of a purple seashell bra and tail, as a result of the Pearl
- Robin (Scott Menville)
- Raven (Tara Strong) - her mermaid form consists of a blue seashell bra and a black tail with blue fins, as a result of the Pearl
- Jinx (Lauren Tom) - her mermaid form consists of a black seashell bra with violet streaks and a black tail with violet fins, as a result of the Pearl
- Cyborg (Khary Payton)
- Beast Boy (Greg Cipes)
- Bumblebee (T'keyan Crystal Kemah)
- Aqualad (Will Wheaton)
- Jean Grey (Venus Terzo) - her mermaid form consists of a green seashell bra and a blue tail with orange fins, as a result of the Pearl
- Cyclops (Kirby Morrow)
- Nightcrawler (Brad Swalie)
- Shadowcat (Maggie Blue O'Hara) - her mermaid form consists of a blue seashell bra and tail, as a result of the Pearl
- Rogue (Meghan Black) - her mermaid form consists of a green seashell bra with yellow streaks and a green tail with yellow fins, as a result of the Pearl
- Gambit (Alessandro Juliani)
- Beast (Kelsey Grammer)
- Wolverine (Keifer Sutherland)
- Colossus (Michael Adamthwhite)
- Mera (Kristin Bauer) - Wife of Aquaman who acts as a mentor to the mermaids.
- Aquaman (Scott Rummell) - rivals with Namor.
- Vulko (Jim Byrnes) - mentor of Aquaman.
- Tula (Melenie Minnichino) - sacrifices her life to save the merfolk.
- Namor (James Arnold Taylor) - rivals Aquaman in many things.
- Namorita (Kimberly Brooks) - Sister of Namor and good friends with Kitty.
- Leviathan (Fred Tatasciore) - one of the 2 main antagonists, A sea dragon who has been dormant for years and is the mortal enemy of the Kraken, his goal is to cause a big tidal wave and exact his revenge on the human race.
- Kraken (Dee Bradley Baker) - one of the 2 main antagonists, A giant Squid like monster that has been dormant for years and is the mortal enemy of Leviathan, his goal is to flood the world for his army to conquer the planet.
- Ocean Master (Richard Green) - the secondary antagonist, revealed to be Attuma's disguise to throw Namor off.
- Attuma (Daniel Wu) - the tertiary antagonist, He is the one who freed the Kraken and the Leviathan at the beginning of the film, and is revealed to be disguising himself as Ocean Master.
- Sea Demons - the quaternary antagonists, servants of the Kraken.
- Sea Snakes - the quinary antagonists, creatures that attack the group inside the caverns.