You put in tidal generators, then you affect the coastal profiles, altering erosion patterns, ecological habitats etc... The materials for solar panels gotta be mined (and that ain't that friendly). Hell, I've (believe it or not) just started a PhD (in Tamaki to rhyme (Tah Mah Key)) on an environmental problem associated with binary geothermal powerstations, which are meant to be almost as clean as you can get (but it turns out everything got it's own problems). Even wind farms, I can't help but feel that if you take the energy out of the air, you'll pay somewhere downstream, it's the nature of the beast.

Oh yeah, and water vapour happens to be one of the strongest (read: worst) greenhouse gases. If you don't beleive me, compare the temperature on a clear night compared to that of an overcast night (you'll notice the difference best in summer naturally). Water's not currently modelled in greenhouse models though, it's just way too complicated mapping it.