Gills

The character grows a pair of gills on the side of her upper neck, just below the jawline, and while she retains her lungs, they are no longer capable of extracting the oxygen she requires to sustain herself. Instead, she is now “breathing” water.
In order to survive, she must find a significant body of water, such as a river, pond, lake, or sea. While a character stranded on dry land could certainly buy herself an extra five minutes by sticking her head into a bathtub full of water, she couldn’t subsist on it for much longer than that (unless the bathtub was fitted with a water recycler to re-oxidise the water and remove excess carbon dioxide). A bucket of water might yield enough oxygen for the character to take the equivalent of a single breath; after that, the water would have to be replaced.
If fitted with a custom-made water-filled helmet that recycles the water, she can still venture into the atmosphere, though she obviously cannot easily communicate with people while wearing the helmet.
A more advanced version of this mutation bestows amphibious qualities upon the character.
Type: Major negative.
Effect: The character now requires water to survive the way she formerly required air. A character who mutates into this form on dry land with no water in sight is in mortal danger. She risks suffocation exactly the way a normal human would when underwater. However, she can survive underwater indefinitely. If the character is lucky enough to undergo this mutation twice, she mutates further and now becomes an amphibian, able to function on land as well as underwater without technological assistance. However, she now must return to water in order to breed, and can only procreate with beings who also have Gills through mutation. Note that when this enhanced condition is granted by biotech, it is bestowed by a separate biodevice that can only be applied to a character who has Gills through mutation or biotech.
Biotech: Implant, Graft.