This helped my nostalgic craving for those pedantic old school HN comments which I seem to see less of nowadays...But frankly as a kid playing in the pool often revolved around pretending to be some kind of sea creature.
Reminds me of the scene in The Social Network (2010) where Mark Zuckerberg points out the absurdity of playing a looping video of the Niagara falls in the background during a Caribbean themed party: https://www.youtube.com/watch?v=N9R8uYUXLUc
The obvious answer is because the old, crudely drawn sign did (and to go deeper, you'd have to ask the janitor who drew it.) Now it's become completely detached from its origin and pointless.
The only reason this question is answerable is because the poster saw the old sign. When everybody is gone who saw the old sign, they'll just continue doing it for no reason. When somebody asks about it, some creep will babble about Chesterton's Fence.
Preschools decorate lists with little pictures on the sides. In most preschools that I’m aware of the person who makes that list makes minimum wage and has 4-8 children to keep alive while she’s making that list.