“Gill Girl” is episode eight of season two of Dark Angel.
Some fisherman find a girl’s body in their net – only she isn’t dead. Instead, she has gills that allow her to breathe underwater; she’s another one of Manticore’s escaped transgenics.
They sell her to a bar owner where she proves to be an attraction (basically touting her as a mermaid). Alec, who is visiting the bar with Normal and Sketchy, spots her and realises she is a fellow Manticore experiment. He asks Max for help in getting her free. Unfortunately, White has heard about her too, and is also hunting her down.
Max and Logan have a nasty scare when it seems like she’s managed to infect him with the tailored Manticore virus again.