I had a friend in high school that had a car for wheelchair users he bought cheap. There was a brake/accelerator stick on the steering wheel. He used to drive with his feet hanging out the window to scare people.
In all honesty, I could find reasoning either in visiting friends while in human form OR possibly to visit deep surfaces that even mermaids can't survive, due to the pressure.
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Maybe it’s like in Splatoon, how the inklings and octolings evolved out of their ability to breathe underwater?