I'll start by saying my information is dated by ~3 decades when I was in HS, so things may well be different, but I suspect not completely.
I swam for Skyine HS is Dallas, we shared the pool with Bryan Adam HS. Skyline coach had been an Olympic qualifier. That's to say, for a poorer inner city school Skyline was in great shape.
Dallas Carter, Kimble and similar schools were in our division. This is not me mocking at ALL. There were competitors that sometimes struggled to finish a 100 free. By struggling i mean, some walked parts or just got out of the pool. It was clear that some had barely been in a pool, ever, much less practiced regularly. I was a 16-17 y/o complete asshole and I felt truly bad for them. Let that shit sink in.
So, I'm absolutely certain there are entire cities worth of amazing athletes that will go unknown for their swimming. Truth is, this isn't limited to swimming, but it's exacerbated by the fact that I don't think a lot even have access to a swimming facility. That's not to even speak of swimming coach/gear etc. The fuck you gonna buy swim goggles in da hood and Phelps ain't driving to South Dallas to coach. (No knock on Phelps, I ain't either)