Press the green flag twice. Arrow keys to move. When you get a weapon press space to fire. Go to the yellow orb looking things. Those are doors sometimes the rooms will look the same just keep going into rooms avoid the red spikes and try to fight the boss at the end. The ey variable is how many doors you've gone through. If you dont finish the game before the timer hits 200 you lose. So really this guy's got a big air tank. Still I gave you enough health to be clumsy and fast so you should be good.
The player is from deep sea diver. The shark is from https://forum.unity.com/threads/darkness-revealed-last-dive-an-underwater-platformer-thriller.389254/page-2 and the music is from https://www.youtube.com/watch?v=V4mMy1hZIpA