Interesting bit of a rant at File770 by poster Darren Garrison:
According to Wikipedia, an average diver needs 1 liter of oxygen per minute. The amount of oxygen that can be dissolved in water depends on the temperature and pressure. According these tables, fresh water at a comfortable 30° C and 1 atmosphere can hold 5.9 ml/l of oxygen. This means that the “average diver” would need all the oxygen from around 170 liters (45 gallons) of water every minute. The highest oxygen saturation on the chart is 40.9 ml/l at 0° C and 4 atmospheres, so the diver would need all the oxygen from around 24 liters (6.4 gallons) of that water every minute. Also pilfered from Wikipedia, I see that the typical tidal volume (amount of air taken in/expelled from the lungs with each breath) for an adult human is around half a liter, and the average adult breathes around 12-20 times per minute.
For the sake of simplifying things, let’s call it 16 breaths per minute, and 8 liters of fluid per minute pass through the lungs. So for this “average diver” to get enough oxygen, the comfortable water would need to contain around 21 times as much oxygen as the saturation point, and the fatally-cold, high-pressure water would need around 3 times the saturation amount of oxygen. And that is assuming that the lungs could extract 100 percent of the oxygen from 100 percent of the water that entered the lungs with each breath. Actual extraction numbers would be only a fraction of that, and thus dissolved oxygen amounts would have to be even higher.
The people would have to be made cold-blooded and live in bone-chilling water (needing less oxygen but becoming very slow thinkers—want to have the sloths from Zootopia making sure your spaceship remains safe?), or trail along large, lacy, and very venerable external gills, or find a way to make oxygen extraction by the lungs vastly more efficient (and in the process make it impossible for them to breathe air, because the amount of oxygen in “normal air” would suddenly be toxic to them.) And you would still have to deal with the issue of releasing waste carbon dioxide.
So saying “it’s easy—just make someone who can breathe water!” is akin to saying “it’s easy—just invent a room-temperature superconductor!” or “it’s easy—just hang a cable that is 22,000 miles long!” or “it’s easy—just assume a spherical cow!”