Indiana/Bella is there because Emma is filming a different film, so she is too busy to do the h20, so in the show she is 'traveling the world!' She may be in the 4th season and the 'h20 movie' if there will be one! Hope this helped!
Indiana as Bella is in the new series because Claire was busy at the time filming the Messanger 2 in Bulgaria. Emma is going to be in the offical movie and maybe if there a series 4 she will come back.
Well, the actress that plays emma was offered a role in another tv show and she accepted it so they had to get a new mermaid and storyline because she left. personally i like bella better than emma and all the other mermaids