The Ambergris Element

Filmation cel from the Star Trek Animated Series episode "The Ambergris Element"

Plot: Kirk and Spock are lost from their survey party when their aquashuttle is attacked by a giant sea creature. They go missing. After a lengthy search, the two are found, mysteriously transformed into water breathers. In order to return to their normal selves, Kirk and Spock must seek out the intelligent life forms responsible for their transfotmation and encounter a group of Aquans.