So I was scrolling through this bulbapedia article, and I found out Zubat is on the Seafloor cavern. Is there any reason behind that?
And I know there are caves in there without any water, but how they come there is something to ask, right?
It's pretty simple. Inside the seafloor cavern, there is air. Thus, a Zubat is able to breathe and fly into you. Maybe in the past, someone released two Zubats and they multiplied like hell.
Other than that, it's probably just a filler.