First of all, this concept is GREAT. I'm not really a fan of this sort of style ( cartoony) but my love of the concept overrides it.
The background is pretty nice, but I don't like how you go from block shading for the most of it, to a softer style on the waves.
You need to work on your lines, in some parts They don't flow, they are angular ( but not in a good way) and they are 'jittery' the outside of the right shark is good, the inside bit is okay, but the lines on the left shark are prime example of what I'm talking about.
The splash affect on the person looks out of place aswell.