Oyoge Yutaro
Retro Pop Culture

Oyoge Yutaro (Swim Yutaro) is a Japanese toy from the 1980s, which was fairly simplistic. It was a very simple "toy" that basically amounted to a plastic plate cut into the shape of a marketable character or fish. The toy was cut in half so there was no back or bottom, and when pushed underwater in the bathtub, the hollow part would catch the air and then force the toy to "swim" back to the surface on the trapped pocket of air.

Relevance to Ranma ½

It's hard to say for sure, but Yutaro, the anime-only character who owned a hot spring, is most likely named for this little product.