Okay, so I've done a first thumbnail for a fish:

It's what you'll see in your logbook when you find these guys, with a brief description.
Right now I'm wondering how I can implement the flora and fauna; I'd like to point out that the diver can't attack, but can be attacked.
How do the accounts turn out? I'll give it a think
What I know for sure is that the radar will reveal their position... I'll make a couple of sprites and see how they integrate to the mock-up.
You start in the Mesopelagic Zone, also known as the "Twilight Zone".
The point is to go further down, through the Bathypelagic, aka as the "Midnight Zone", the Abyssopelagic Zone, aka the "Abyss", and finally the Hadalpelagic Zone, the "Trenches". In order to go further down, you have to upgrade your suit to withstand the increasingly high pressure!
The diver you play as is disgusted by the world he lives in, and chooses to withdraw himself in the depths of the sea. He wants to contemplate its beauty whilst denying the reality of above. This state of denial, but also his detachment from reality, is reflected by his progressive descent. As he goes down, pressure-induced (and solitude-induced) fantasies of underwater cities, mermaids, etc. mark his way to the unseen underworld of the Trenches.
Here's a second fish thumbnail:
