The Only Water in the Forest is the River

Speculation again! What on earth does this mean, we get that River means River Song but what does the forest mean and what does this whole thing about the Water mean?

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?

Greg