Keep swimming down, even in the darkness. Keep holding right click non-stop. I hope you can finish the game!
Oh not sure why I couldn't advance last time. Thought I did hold right click. Well anyway, made it to the final boss and only got him down to half of its hp. I'll try again later though. Also edited my first comment
Keep going! btw you can deal extra damage if you hit all 4 eyes of the boss!