Right, My theory (it may be too far fetched for some, but still) is that the forest is a metaphor for the TARDIS, and that what Idris was saying is that 'River Song is the only 'Water' in the TARDIS'. Which, twisted around to be less riddle-like. Could mean something like 'Amy Pond isn't real, but River Song is'. I don't know, too far fetched? I already have my theory that River is Amy's daughter, but if I was somehow right, what does this have to do with Idris quote?