My castaways on Saturn live on fibrous organisms that extract electricity from the turbulence of the atmosphere. They have chosen to adapt to the planet by implanting electrolysis equipment that converts water to hydrogen and oxygen from an internal battery that they can recharge from those organisms. This permits them to "breathe" in Saturn's atmosphere.
The electrodes end in a mesh similar to an inferior vena cava filter, on which are mounted many microvilli on which a water splitting enzyme (a highly simplified and very stable version of photosystem II is exposed. The electrical current entering the filter powers these enzymes. Blood is not directly exposed to the electric current -- the enzyme ensures only water is reacted. For convenience I will call this the 'internal gill'.
The internal gill is in the portal vein, which passes blood from digestive organs to the liver. This gives an extra layer of protection against potential clotting at the gill, and delivers oxygen in the same manner is it is received during fetal development.
The castaways inhale mostly H2 at 15 atm of pressure with a few impurities (dealing with alkaline pH and H2S is a separate issue). They exhale H2 with a small (40 torr - see table) partial pressure of carbon dioxide. A trace of oxygen also gets out, but on Saturn this is not nearly enough> to support combustion.
This system allows people to breathe the right partial pressure of oxygen and exhale the normal amount of CO2 and all is great, I hope, except ... I can't expect castaways on Saturn to invent this from scratch! With this sort of progress you always have to have a certain learning curve, and there aren't enough of them to survive the curve.
While the system might be used elsewhere in space (I made an answer about this regarding Mars), here we're still quite early in the Conquest of Space and we need an Earthly antecedent for the invention of electronic respiration. For now I'm going to say that this invention was by remnants of the Navy Seals, who organized New Atlantis off the California coast after the disintegration of the U.S. What began as a shallow-water network of submerged safe houses for exfiltration of refugees and smuggling ended up moving much deeper. To enjoy the freedom of the seas, the divers perfected the internal gill, and with it, pioneered a new society.
On Earth, producing 66% hydrogen and 33% oxygen, most but not all of which is used up and exhaled as carbon dioxide, has some problems. It might be flammable if enough oxygen gets through, though I think it would have to be 25% or more so that may not be a huge issue. The bigger problem is that it is hard to carry out the CO2. If the lungs are 1/3 filled with CO2, partial pressure something like 250 torr instead of 40, and increasing with depth to colossal levels, the person will be in a lethal state of acidosis. Diluting it with nitrogen isn't an option due to nitrogen narcosis. Hydrogen is sort of an option, but splitting that much water would waste most of the battery power, and flammability becomes an issue for sure.
One option is to give up entirely on having lungs exhaling gas. Filling them with sea water doesn't sound good, but if they are modified with a few genes to produce alpha-fetoprotein (an albumin-like substance) and sodium and chloride channels, they could maintain themselves with an amniotic fluid like substance unless and until the transgenes are deactivated.
I'm also going to allow myself one Fact Not In Evidence and say that when there is fluid in the lungs and oxygen in the portal vein, the medulla instinctively "realizes" there is a fetal circulation pattern, even in an elderly adult. The person loses the impulse to breathe and feels perfectly comfortable never doing so. There is no evidence to support that, but the notion of the Seals giving themselves Ondine's Curse to survive in their habitat is too extreme. (They could do it, with focused ultrasound to the medulla ... let's not go there)
But now -- what to do with the CO2??? [Response: The carbon originates from food. Food energy could be provided by hydrogen given the right modifications, but this starts with humans requiring a wide range of essential nutrients.]
- It can be converted to bicarbonate and excreted in the urine - with just the right level of bicarbonate vs. urea or even carbonic acid the pH can be controlled, and the kidney tubules can already do that (if in far lesser quantity). The problem is, this works out to secreting kilograms of bicarbonate daily - it's just not going to be feasible to drink water and produce so much filtrate.
- Or we can picture CO2 being bubbled away from the skin. Maybe a protein actively excretes it, at a considerable cost in energy. But we're talking about as much CO2 bubbling away as in 1/19 of a breathe, in the span of time it takes to breathe. It seems hard to do.
- Or, we can go metabolic. Convert the CO2 into some kind of food, and then the electrical energy is not being wasted in H2 but recycled. The problem is, CO2 goes to formic acid, formaldehyde, methanol, methane ... it's not the most palatable menu. We could convert it to formaldehyde then polymerize that to carbohydrates (glucose or phosphoglycerate derivatives) but that seems quite complicated, especially at such an early point in history.
Help me sand the rough edges off this. How do we make electronic respiration in deep seas of Earth as plausible but simple as possible, especially regarding CO2 clearance? And ... how implausible is that?