I think the other visions Desmond could "see" becuase he had been there and was having kind of a Deja Vu "memory." My bet is that Desmond comes down and helps... hence his having seen it.

Flipping the switch might cause it to flood.

I don't think we saw his first few flashbacks.... Charlie drowning, him knowing where the bird was, lightning striking, Locke making his speach...etc...