I really enjoyed the graphics. The bright cartoony colors were really pleasing. The distinction between the background and the animals was also nice. The jellyfish was really having a lot of fun from the looks of things.
The animations are a bit odd. They looked a bit like the duck was flying. Also for the swim-up and swim-down animations they didn't trigger frequently. They only triggered if you had 0 horizontal speed. It would be better if they would get triggered when velocity's vertical component was higher than the horizontal component.
The music was really nice. The sound effects did get quite repetitive. When you have sound effects that play frequently you can either make multiple variations or - that's the easier option - randomize the sound effect pitch (for example between 0.9 and 1.1 times the original pitch).
Also, not sure what was up with the sharks. They only spawned from one spot and in one direction. That didn't really make them all that difficult to beat.
Additional points for including a duck as the main protagonist +1. Good job!