A simple reason could be that she was exploring a new and dangerous part of the ocean. There might be hidden rocks or strong currents that she wasn't aware of, and she could get banged up against the rocks or get exhausted from fighting the currents.
She could get hurt because of a rival mermaid group. If there are other mermaids who are jealous of Mimmi's powers or her position in the pod, they might set a trap for her. For example, they could lure her into an area full of poisonous sea plants that would sting her and make her ill.
Perhaps Mimmi gets hurt when she ventures too close to a human - made underwater structure that malfunctions. It could be an old, rusty oil rig with sharp edges. As she swims around it out of curiosity, a piece of metal could break off and scratch her tail, causing her pain and making it difficult for her to swim back home.
He could be exploring an old, abandoned building and fall through a rotten floorboard.
One possible plot could be that the characters from Mako Mermaids end up in a different magical realm where they meet the mermaids from another story. They could team up to face a common threat, like an evil sea witch who is stealing the magic from all the mermaids. They would have to combine their unique powers, like the moon pool powers from Mako Mermaids and the special singing abilities of the other mermaids, to defeat the witch.
One reason could be that he was being a bit too adventurous. He might have tried to climb a tree that was too high or unstable for him. As he reached for a higher branch, he lost his balance and fell, getting hurt in the process. Also, bad weather conditions can play a role. If it had rained recently and the ground was slippery, Chris could have slipped and fallen while walking in the park.
One possible plot could be that Zac and Lyla are on a mission to save the mermaid realm from an evil force. During this dangerous journey, they grow closer and closer. The kiss could be a moment of celebration after they succeed in their mission, a way to express their relief and newfound closeness.
One possible storyline could be that Zac and Lyla's pregnancy is a result of a powerful mermaid magic gone awry. Maybe they were trying to perform a spell to protect their underwater world, but it had an unexpected side effect on their relationship and led to the pregnancy. This could bring a lot of drama as they have to deal with not only their personal feelings but also the reactions of the mermaid community.
Well, the mermaids could be brought to the surface world due to some sort of magical or scientific accident. And the Avengers, being the protectors of the Earth, would initially be cautious but then realize they can be allies. The mermaids might have some sort of ancient magic that could be used to power up the Avengers' weapons or defenses. Or perhaps there's a common enemy that is after both the mermaids' underwater resources and the Avengers' technology. So they have to team up to stop this greater threat.
Maybe a magical item gets lost that is crucial to both the H2O and Mako Mermaids' worlds. It could be a pearl that has the power to keep the balance of the ocean's magic. A young mermaid from H2O and a young mermaid from Mako accidentally cross paths while looking for it. At first, they are suspicious of each other. But as they search together, they start to understand each other's powers better. The H2O mermaid can freeze water to access hidden caves, while the Mako Mermaid can use her singing to call upon sea creatures to help. Eventually, they find the pearl and in the process, they create a friendship that bridges the gap between their two worlds.
It could be a failed experiment in potions class. Hermione is always eager to try new things in potions, and perhaps this time it backfired, causing her some harm. And Snape would be the one to find her because it's his class.
A common reason is battles. Sasuke is often involved in intense combat situations in fanfiction, whether it's against old foes with new powers or entirely new enemies. His determination to prove himself or protect something often leads him to take risks that result in getting hurt.