Many areas and chests are unavailable because they are blocked by a sea of water. The only way to reach these areas is by unlocking a character that can swim.
How to swim
To start moving around in water you need to unlock a water character. There are multiple water characters in the game but they are unlocked pretty late in the transformation tree. The earliest water characters you can unlock are the turtle and the mermaid. They require the same level to unlock, but the turtle is in the left branch and mermaid is in the right branch. After you transform into a water character all you have to do is move into the water to start swimming. There are no quest or level requirement.