While H2O: Just Add Water had three seasons, it's not the end of that universe because it spawned a spin-off, titled Mako Mermaids, also known as Mako: Island of Secrets.
The series premiered 26 July 2013, and although I've not watched it myself it appears to be a flipside of the adventures of H20 where instead of a trio of human girls turning into mermaids, there are three mermaids who get a chance to gain legs and figure out what being human is all about. There's also a human fella who gets in on the transformation action, so there's plenty of opportunity for wacky antics and culture clash bonding. Trailer below!
Thanks to Wong for the head's up on this new show.