To get all of the chaos emeralds beat jhonny in all 7 races scattered throughout the sea map they are extremely harder as you go on.
Cool stuff
Sometimes people will award you with all sorts of decorative items for your island however you will have too complete missions or pay rings for them.
Deep Core
To unlock Deep Core you need all 7 chaos emeralds and sol emeralds defeat the ghost titan and it will appear at the final chapter and you unlocked it.
Muzy (Sound Test)
After defeating the Titan boss in Big Swell, go to Seagull Beach on Windmill Island and talk to the koala with the guitar. You will now be able to listen to music, sound effects and dialogue from the game, and can receive missions which unlock extra sounds once completed.
Play as Blaze
Defeat the Kraken Boss in World 3 (Coral Cave) to play as Blaze the cat.
Sol emeralds
To get the sol emeralds discover and complete all islands at least once then talk to the captain he will tell you to inform blaze that one of the emeralds have been located. You will either have to finish a race or rebeat the bosses.
Sound test
Defeat the Titan Boss in Big Swell. Then go to Seagull Beach on Windmill Island. Talk to the koala with the guitar to hear the music, dialogue, and sound effects. This will also open additional missions.
Super blaze + super sonic
To get super blaze and super sonic get all sol and chaos emeralds and you will fight the final boss inside the islands massive volcano.