The Octonauts and the Little Mermaid is the 20th episode of Season 1.
When Kwazii and Dashi meet Ariel the little mermaid, they try to convince Captain Barnacles, who doesn't believe in mermaids, that mermaids do exist, but when Kwazii senses that Atlantica is in danger, he and the Octonauts come to the rescue. After the big save, Triton, Ariel's father, awards Kwazii with a special ocean pearl bracelet.
The episode starts in the Octopod with Kwazii playing table tennis with Dashi and Inkling when he sees a blurr of red, green, skin peach, and ruby red pass by which had him distracted from the game and he and Dashi go to the window to see what it is, but it disappears. So they go outside to find the creature who swam to the coral cave filled with treasure and lost things. Just then, they felt something swam right behind them very fast as Dashi turned on her flashlight to see who or what it was. When the light stops, Kwazii and Dashi come face to face with...a mermaid named Ariel!
Captain Barnacles: Look. You made me and Peso believe in fairies when you were in Never Land, Kwazii, but I'm not sure you and Dashi will make me believe in mermaids as well.
Kwazii: Aye, but mermaids do exist captain! Dashi and I just met one, which means that mermaids do exist! Because-
Captain Barnacles: Enough! We'll talk about this in the morning but right now let's get some sleep. That way, it'll calm your imagination about sea monsters and mermaids.
Kwazii: But- (stops and groans) Fine!
(That night, Kwazii was in bed sleeping when suddenly, he starts to have a nightmare about an upcoming sea tremor that made Atlantica shake and crumble. He could hear merpeople screaming and swimming for their lives! He sees a mermaid's tail stuck under a rock when a broken baloncy begins to fall on her. Swimming fast towards her, Kwazii freed her and helped her escaped and ends up getting crushed! Kwazii is then woken up by the real sea tremor just like in his dream)