Ok - here is a brief update:
I'm currently still busy building the game world. I'm almost done doing so, though it has taken longer than expected. Unfortunately, UV unwrapping/mapping feels like rocket science :)
I'm a bit conflicted about the character controller. The current one uses a standard Mouselook script, along with a functionality that adds a (decreasing) impulse force in the direction the player is facing, basically simulating a swimming stroke. However, divers usually have flippers, which makes this mechanic inaccurate. Nonetheless, I do think it would be more interesting this way.
Anyway, I'll make a bigger update when I'm done with modeling.