It seems that at least in the Andes, a lot of people say e.g. [awa] for "agua"[agwa]. What's the phonological rule behind this? Is it really [w]? Why did this happen in the first place?
At least from my experience as a native speaker (of Chilean Spanish), I think it's a more popular thing to hear that, as an example, I recall the word for someone with a big belly is
guata = belly), which, as it's used in more informal contexts, could be heard as
watón, while it should be written
Hope it helps.
In the title line you speak of "intervocalic g", while you actually mean "g followed by u", which is different. Only the latter case bears a similarity to "w".
In all languages I know, speakers make a difference between careful speech and casual speech, and this I believe is the case hier: "aɣwa" is careful speech and "awa" is easygoing speech.
I would also keep in mind that for millions of people in Peru and Bolivia Spanish is just a second language, so I would not expect everybody's pronunciation to be fully academic.