Ok so i'm not sure about any of this but this is what I've heard.
*Emma moves away to film the messengers 2 but is in the first episode
*Lewis disappears after a hurricane
*Rikki and Zane breakup :(
*Rikki likes Will who accidentally unleashes powers of makko island and turns the water against the girls
*Bella, a mermaid since she was 8, is a singer who wants to date Zane but Rikki won't let her (so maybe this means she still likes him!!!!)
*I have heard different things about Bella's power: I have heard they give her Emma's power and that she can turn water into slime... kinda weird...
*Zane is heart broken and is determined to revel makko island's secrets
*Season 3 premieres in October on channel 10 in Australia
That's it I think add more if I missed something