>>122035074In the episode, "sardine" level swimmers are beginners in the class. When they graduate, they become "scorpion" level swimmers.
The joke is that they went from a lame animal, but one that can actually swim (sardine) to a cooler animal, but one that can't swim at all (scorpion) as a symbol of what's supposed to be their increase in skill.
It's a very American joke, since kids swimming lessons over here often use arbitrary "ranks" like minnow, guppy, whale, dolphin, etc. "Scorpion" is such an absurd choice that it just adds to the joke.
