Ocean Gate

Ocean Gates are magical portals that are located deep underwater of each planet in the Magical Dimension. Each gate is guarded by a Selkie and an underwater triton guard.

Appearance
The gates appear as two sharp rocks, that almost forms a circle, leaving an empty space between their upper spikes. With Selkie magic or with incredibly forceful magic, the gate glows brightly and the portal opens. They look the same from Andros side. But from the side of their worlds, they have features, resembling to a element of the world.
 * Solarian Ocean Gate is made up of two sharp rocks with some barnacles.
 * Melody's Ocean Gate is shaped like harp from Melody's side.
 * Zenith's Ocean Gate is dark colored and has circuital lines and circles on it.
 * Linphea's Ocean Gate is blue, shaped like leaf and some green seaweed is wrapped around it. In the comics, it is a U-shaped portal made of green undersea rock.
 * Domino's Ocean Gate is dark and curly, forming an C-letter like shape, that also looks similar to flames.
 * Infinite Ocean has a massive gate, which the other side is possibly located in Andros. It does not have guardian Selkie, but it requires powers of each other existing Selkies to enter. Once it is entered, one does not need Ocean Gates anymore as the Infinite Ocean itself grants access to any existing ocean in the magical dimension, including Earth.

Known Gates

 * Gate of Andros: Located near the Prison of Andros and is guarded by Lemmy.
 * Gate of Earth: Located in Gardenia, near Paradise Bay and is guarded by Phylla.
 * Gate of Solaria: Located near the Azure Valley and is guarded by Illiris.
 * Gate of Melody: Located near the Ruby Reef and is guarded by Sonna.
 * Gate of Zenith: Location near the Data Bridge Castle and is guarded by Lithia.
 * Gate of Magix: Located near Lake Roccaluce and is guarded by Nissa.
 * Gate of Linphea: Located near the Ocean of Flowers and is guarded by Desiryee.
 * Gate of Domino: Located near the Yellow Reef and is guarded by Serena.
 * Gate of Infinite Ocean (Sirenix Gate): Can be summoned everywhere underwater by Sirenix fairies/Dark Sirenix witches and is not guarded by any Selkie.
 * Gate of Oblivion: Can be summoned anywhere only with immense magical energy.

Season 5
At Gardenia, Earth, an oil spill occurs and the toxic blends with magic, opening the portal to Andros. The magic pollution turned Tritannus into a monster, who can mutate sea creatures and enslave them. Escaping from the Prison of Andros, Tritannus finds Phylla and Lemmy, the Gatekeepers of Earth and Andros, respectively. To refuel, Tritannus enters the Portal of Earth and absorbs some toxic. Tritannus returns to Andros to continue his plans. He sends out magic pollution to the Labyrinth of the Abysses. Then, he absorbs Illiris, Solarian Gatekeeper's, magic. He enters Solaria and pollutes the Royal Deeps. He goes to the Courtroom of the Underwater Palace of Andros to mutate Neptune and Tressa, and he succeeded. In the process, he also takes the power of Nissa, Gatekeeper of Magix. He, then fights Gatekeeper Sonna and rips her off her magic. He enters Melody and transmute Musa. After that, he goes to Magix and abducts Daphne. He also sucks the power of the Gatekeeper of Zenith, Lithia. He mutates the triton guards in Data Bridge Castle.He continues his quest and rips the magic of Linphea's Gatekeeper, Desiryee. He enters the Ocean of Flowers and pollutes the Flower of the Depths. Next, he takes Serena, Gatekeeper of Domino's, power. He pollutes the Great Yellow Reef. After taking all the power of the Selkies, he go through the Magix gate and destroys the Source of Sirenix in Lake Roccaluce. He rips Daphne's Sirenix magic off her and gives it to the Trix. Same day, Omnia lets the Winx enter the Sirenix Gate.

Now that Tritannus has entered Infinite Ocean, he does not need Ocean Gates anymore as ocean itself grants him direct access to any ocean in magical Dimensions and also Earth, which is connected to it.

Season 5
Tritannus uses the Linphea Ocean Gate to traverse the realm's oceans in search of the Roots of the Shimmering Flowers with three mutants by his side. It turns out that he plans to suck the lifeblood of the realm's plant-life from the roots, leaving it all to die as its oceans turn to mud.

Trivia

 * Ocean Gates were indirectly mentioned by Aisha in Season 3 as she told the Winx that all oceans in the Magic Dimension are magically connected.
 * It is safe to assume that she uses one in "The Princess' Ball," when Tala calls her back to Andros.
 * It can be assumed that every world with some kind of an ocean has an Ocean Gate and guardian Selkie.
 * An Ocean Gate can be activated to open in any other realm and is not limited to to the realm to which it is associated.