"The wet shoe is the one that was in the water."

Translation: La scarpa bagnata è quella che era nell'acqua.

[K.Alix]

"La scarpa umida è quella che era in acqua." Why is that wrong?

[cronistamarie]

I think because you're missing the definite article before "acqua." "In acqua" = "in water" vs. "nell'acqua" = "in the water."

[Biomax]

I would say they are almost the same. Your sentence is grammatically correct but, as already stated by cmccluskey, you forgot the definite article. Also, there is a slight difference in meaning between "scarpa umida" and "scarpa bagnata". It it's like if "scarpa bagnata" had a little larger quantity of water than "scarpa umida"