If you grew up in the mid-2000s, there’s a high chance you spent your Saturday mornings wishing you could sprout a tail after a quick splash of tap water. H2O: Just Add Water was that show. It was peak Australian teen drama, mostly because it managed to make the ridiculous idea of three girls turning into mermaids on Mako Island feel kinda... relatable?
At the center of it all was Cleo Sertori. She was the shy, slightly awkward one who was initially terrified of the ocean. Weird choice for a mermaid, right? But it worked. If you’re here, you’re likely asking who played Cleo in H2O, and the answer is the powerhouse that is Phoebe Tonkin.
Honestly, it’s hard to imagine anyone else in those purple sequins. Phoebe didn't just play a mermaid; she launched a career that would eventually take her from the Gold Coast to the dark, moody streets of New Orleans in The Originals.
The Girl Behind the Tail: Phoebe Tonkin
Phoebe Jane Elizabeth Tonkin was only 15 years old when she landed the role of Cleo. Think about that. Most 15-year-olds are struggling with algebra, but she was learning how to swim in a 40-pound silicone tail.
She actually admitted later that she wasn't even a strong swimmer when she got the part. Talk about "fake it 'til you make it." She spent hours in pre-production training just so she wouldn't look like a drowning human whenever the cameras rolled.
Phoebe's portrayal of Cleo was special because she nailed that specific teenage insecurity. Cleo wasn't the "cool" girl like Rikki or the "perfect" girl like Emma. She was the one who was stressed about her annoying sister Kim (who was truly the villain of the series) and her messy feelings for the lovable science geek, Lewis McCartney.
Why Cleo Sertori Still Matters
Cleo's arc over the three seasons of H2O is actually pretty deep for a kids' show. She starts as someone who literally can't swim and ends the series as a confident young woman who basically takes over as the group's "brain" once Emma leaves.
Her powers were always the coolest, too. While Rikki could boil water and Emma could freeze it, Cleo had hydrokinesis. She could mold water into shapes, suspend it in mid-air, and eventually, she could even control the wind.
- Season 1: Shy, water-phobic, and totally dependent on Lewis.
- Season 2: Dealing with the "Charlotte drama" (we all hated Charlotte, let’s be real) and growing more confident.
- Season 3: A literal leader who excels in science and keeps the new girl, Bella, in the loop.
People often forget how much Phoebe Tonkin brought to the physical acting of the role. Using "magic" on screen in 2006 wasn't about high-end CGI; it was about Phoebe waving her hands in a very specific, fluid way to make the audience believe the water was moving.
Life After Mako: Where is Phoebe Tonkin Now?
If you think Phoebe’s career ended when the scales fell off, you’ve been living under a rock. She’s arguably the most successful export from the show.
Shortly after H2O wrapped in 2010, she moved to Los Angeles. It didn't take long for Hollywood to notice her. She stayed on her H2O co-star Claire Holt's couch while auditioning—yes, Cleo and Emma are best friends in real life!
She eventually landed the role of Faye Chamberlain in The Secret Circle, but her true "breakout" in the States was playing Hayley Marshall in The Vampire Diaries. That role was so popular that she became a lead in the spin-off, The Originals.
Recently, she’s been doing much "edgier" work. You might have seen her in the 2022 film Babylon or the 2024 Netflix series Boy Swallows Universe. She’s also a massive face in the fashion world, working as a Chanel brand ambassador. Not bad for a girl who started out working at a fictional juice bar.
Surprising Facts About the H2O Production
You might think filming a show about mermaids is all sunshine and tropical vibes, but the reality was pretty grueling.
- The Tails Were Heavy: They weighed about 18–20 kilograms (around 40 pounds). The actresses had to be carried to the water because they literally couldn't walk once they were zipped in.
- Long Days in the Tank: Most of the underwater scenes were filmed at Sea World on the Gold Coast. Phoebe and the others spent hours submerged, often in cold water, while trying to keep their eyes open and look graceful.
- Real Life Friendships: Phoebe Tonkin, Claire Holt (Emma), and Cariba Heine (Rikki) remain close to this day. In 2024, Phoebe even got married to art dealer Bernard Lagrange, and her former co-stars were right there celebrating with her.
What to Do if You're Feeling Nostalgic
If reading about who played Cleo in H2O has made you want to dive back into the Mako Island mystery, you're in luck. The show has a massive second life on streaming.
- Watch the original: You can find almost every episode on YouTube (the official H2O channel is surprisingly active) or Netflix in many regions.
- Check out "Mako Mermaids": It’s a spin-off set in the same universe. It’s a bit more modern, but it doesn't quite have the same "Cleo, Emma, Rikki" magic.
- Follow the cast: Phoebe Tonkin and Claire Holt often post "The moon is silver" jokes on Instagram, which is basically catnip for old-school fans.
If you really want to see how much Phoebe has grown as an actress, go watch The Originals. Seeing "Cleo" play a fierce werewolf-vampire hybrid is the ultimate trip. It shows just how much range she had, even back when she was just a teenager worried about getting her hair wet.
The best way to appreciate Phoebe's work today is to check out her recent Australian project, Safe Harbour. It’s a psychological thriller that is lightyears away from the Gold Coast sun, but it proves she’s one of the best talents Australia has ever produced.