Horseandadog
Susan and Katie and two ordinary girls, until Katie finds a map leading to mermaids. They collect items on their short adventure to the lagoon where mermaids await them.
They are each given a wish, and you can guess what they wished for.
Now the two girls are going through many Summer-time adventures, as Mermaids.
Will they grow tired of keeping the secret?
Book 1: Summertime Mermaids