We were at Downtown Disney and there was a small little girl, who was white as cotten sheets running in the pop up water fountain completely naked. Her parents were right there watching her, but I guess they were from another country that thinks nothing of this. There was another time at Ariels little water playground where a mom went to take off her small daughter's shirt and a CM told her to put it back on the child.
This is actually quite common in a good number of places at WDW. There are such fountains in MK, Epcot has several, and water misters in AK. AK also has one 'shower' water play area where you pull a cord and get a shower. HS has one small such area that I recall, but it's small and not really a fountain. (It's meant to copy Singin' inthe Rain, so you stand under the umbrella and don't really get very wet.)
Lots of times I've seen small kids playing in these fountains in their underwear or diapers. I don't recall seeing naked kids, but lots of times I've seen kids as old as four or five playing. Sometimes kids have bathing suits, but most often they do not. I don't think I'd encourage my family to do the same, but I've never thought it was a big deal.
Away from the fountains though I'd maybe find kids in underwear odd. Diapers not so much, but underwear yes.
Then again, I've also seen LOTS of folks comes off of Kali and Splash in sheer cotton clothes or the like.You know, the kind of white cotton they use for wet t0shrt contests. Only, well....lots of the folks wearing the translucent fabrics are not in prime physical form. Seeing through their clothing is not really flattering, shall we say.
Oh, and pretty much EVERYtime I've gone inthe restroom next to Kali there's some poor lady standing shirtless or pantless trying to dry her clothes - usually denim- under the hairdryer. Often enough there's actually a line of nearly naked ladies and kids waiting for their turn at the dryers.