Played all 10 levels, I can give some suggestions
You could add skulls that move but will probably need to indicate where they are with ripples in the water
You should add variable scares when hitting the skull as it can get a little old to see the same one each time
The disappearing and reappearing skull mechanic makes it into a maze and memory game at the same time which isn't bad but there isn't any reason to not just wait around for them to reappear. Something things that could amp up the horror factor would be having the skulls watch you or start following you. It might be hard to balance how much time to give the player to memorize the maze with a time limit though.