Actual underage nudity - maybe even more shocking today than in 1938
The stuff in the pool is somewhat less racy than the scenes in Tarzan and His Mate.
BUT.
When she undresses before she jumps into the swimming hole, you can clearly see her breasts and her nipples. It really was unexpectedly shocking even now, given that the actress was indeed twelve when it was filmed. It's not hard to see why MST3K would pass on this one.