I am writing the lore for a currently in-development open-world freediving adventure game and am stuck with trying to keep some things scientifically founded. Overall the game style is closer to realistic than to stylized or cartoony (it's a 2.5D game to make this feasible in a small studio) and the lore should fit this atmosphere.
The time period would be a couple of decades into the future. Mankind has made universal progress, including some artificial, biological enhancements to themselves, but with 10 billion people they do not keep the oceans clean by any means. Warming and souring of the ocean has also drastically influenced its food chains.
Base of the game lore is a number of similar species of sirens coming out of hiding from the depths of the ocean as that got further polluted by humans. While not willing to communicate with anyone at the surface, they seek to reclaim the sea and forbid humans from traversing across the surface, let alone diving deep.
The Sirens abilities/technology
They are a sapient species with a generally inferior level of technology and possibly magic (albeit in terms of magic I'm mainly thinking of body-enhancing "natural" compounds which the player can later use as well).
Their main perk is agility and eyesight/echolocation underwater, making even just rapier-style cutting weapons rather efficient. Using the thousands of water mines humans have deployed over the decades as well as natural resources, they can craft sufficient explosives or corrosives to sink ships and submarines especially after learning of weak points.
Furthermore sea life is clearly on their side (or they breed and train it), allowing them to deploy areas of fish and jellies for alarm/scouts + passive defense and sharks as deadly guards (not as offensive attackers since RL sharks already suffer enough under their reputation as bloodthirsty killers :( ).
The Lore vs Gameplay Problem
Yet with that background of not making the Sirens absolutely overpowered, organized teams of Navy Seals or similar equipped with scuba/rebreathers would have easy game defending themselves and counter attack at least in the Epipelagic Zone (above 200m). Specially designed agile submarines with omnidirectional defense capability (instead of just stealth like today) could go even deeper.
What I am aiming for the game though is that less organized and peaceful freedivers (biologically enhanced to be mostly proof against diver sickness and pressure) are the ones who will manage to discover secrets regarding where the sirens come from, what they truly want and eventually lead to some form of resolution. The gameplay shall be in the direction of Subnautica: Always having to take your air into account and you are able to make air pockets with slow air-generators to secure progress.
Best solution to achieve this would be to make any form of high pressurized air supply (as gear or in submarines) mostly unusable. How to explain that without taking the game fully into magic territory is my problem.
It always irked me in Subnautica why you don't have proper scuba tanks. Sure the "on board" compressor on your suit might be technologically limited, but you'd still bring a regular tank at very least for emergencies which can be refilled at a full size compressor at a base.
I'd like a more proper solution with potential for gameplay elements (like providing POIs).
One thing that crossed my mind would be the sirens possessing a type of crystals or technology that emits a radiation which mainly affects pressurized oxygen, similar to how supposedly microwaves only heat up water. But turns out that on its own, is a myth - it's not how household microwaves actually work.
The land dwellers do need to send humans when they see the situation as a war (which they will, because transporting goods across the sea is as important as today). The issue of wireless transmission being very limited underwater has not been solved, making drones easy targets due to their cable and AI has not progressed further enough to truly replace humans for underwater missions (small AI drones will be partners for the protagonist though).
It does not necessarily need to be an absolute exclusion of scuba gear for everyone. The freedivers will just have a higher chance to not immediately incite aggression by the Sirens as at least some (probably one of the species) will recognize that humans who dedicate themselves enough to dive without gear do actually care about the well-being of the ocean and could be friends like dolphins are.
I am okay with toning down human ingenuity. Like surely one could probably think of drone-swarms that maintain a short-range network via ultrasound to be controlled from the surface without cables and if pressured O2 is really the weakpoint of scuba gear, there are chemical and electrical ways of generating O2. The game isn't at all about military tech however.
Yet I would like to prevent the player from getting the obvious feeling "all this would be so much easier with scuba tanks and some spear guns; no human would risk doing this freediving".
Am happy about any suggestions! Huge thanks!