Because of the pollution in the water the large sea creatures have become confused about what they should eat and now they're eating everything.
 
Save the sea creatures by tapping them to warn them that they'll be eaten. 
 
Allow the starfish through (don't tap them).  They are safely stored in the large creature's bellies to be used as money for upgrades later.
 
Play 35 Levels

Replay any level to get gold (based on the level score and stars collected).

Collect stars to unlock more features:
Unlock two more themes.
Unlock the shark that speeds everything up.
 
Beat your own high score.
 
Choose / switch between casual mode and expert mode.  In expert mode everything moves faster, and you have less health.
 
Difficulty:
It is a quick tap game.
The game may take a few tries to pass in casual mode - but is passable.
Expert mode requires quick fingers.
Playing with the shark in expert mode is for those who like a challenge.