The aim of the game is to stay alive for as long as possible.

A timer is constantly ticking down.
When it, or your health, reaches zero it's game over.

Extra play time can be earned by sinking boats with your attack.
More time is given for sinking the tougher ships, which need to be attacked more than once.

The attack action causes you to charge upwards towards the water's surface.
Attacks can only be performed if there is sufficient distance between your current location and the surface.

If a boat that is capable of attacking spots you, it will fire projectiles. You might consider avoiding these.

Moving inside a patch of seaweed will camouflage you, and the ships will not attack.
It is necessary to align yourself either vertically or horizontally with the seaweed.
If you are camouflaged, this will be indicated by the shark changing colour.

Just like a real shark you will need to keep moving to breathe.
Stay still for too long and you will start to sink, moving you out of any seaweed you may be hiding in.
If you continue to stay still you will eventually begin to loose health.

Good luck and have fun playing!