Star Trek: The Animated Series, season 1, episode 13
The Ambergris Element
Summary
While exploring the water planet Argo, Captain Kirk and Mr. Spock are transformed into water breathers by the planet's undersea inhabitants, the Aquans. In order to return to their normal selves, they must enlist the help of the Aquans to capture a giant sur-snake, whose venom holds the antidote.
Summary from Wikipedia, CC BY-SA 4.0.
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