Good work and interesting concept.
In my first game I didn't encounter any sharks for a long time, so it became a bit boring, however when one appeared I got so surprised I stood no chance. x'D
Personally I would like to have another, easier type of challenge in the game alongside the sharks, to stabilize the tension a bit.
You have a nice base there to add more features to :) Nice trick preempting the danger with music motiff... makes it also feel a little bit like Don't Starve... except there the clock is what preempts the danger.