A few tips for more improvement in the future, aside from the build of the game.
HUB: Despite using assets from Unity, try building your level to be more cohesive and use colliders because I can just go out of bound of the HUB; using the tutorial to guide the player is alright, but the level itself needs to guide the player to the objective.
Diving: The controls felt a little clunky at times, and I think there needs to be a way of refilling your oxygen underwater.
Overal, great first attempt, you just need to work a little more.