So, clearly, you're trying to make this a horror game of sorts. And you've almost got it, the environment is very grim and dark. It's just that the main threat(sharks) aren't really scary, as they just appear right in front of you, and all they do is swim towards you. You also never have a reason to swim upwards, as all artifacts are on the sea floor.
I have a few suggestions:
- Make sharks spawn when you go to down instead of up
- Increase the player movement speed
- Give the player some kind of indicator to know where to look for the next artifact
- Make sharks appear from the dark, and make them something you can actually escape from