What's the connection between the two shows?
H2O and it's spin-off Mako Mermaids.
Because H2O has three regular teenage girls magically transformed into mermaids and it's just them hanging around the Moon Pool with their friends.
While Mako Mermaids has a whole pod full of mermaids that live on the island and hang in the Moon Pool and then they're super scared when a teenager gets transformed into a merman.
So where were all these mermaids with their fear when the girls turned? It's set on the same island. They even said that Cariba Heine from H2O is gonna guest star on the new season of Mako Mermaids.