The atmosphere and lighting is stunning, Andy. Great shot. The water is hardly visible as being water, though. Is it still, I mean no waves? You can probably save a lot on render time if you manage the water well, especially when a cloud layer is partly underwater (you can blend the underwater part out by distribution shader or the cloud layer settings, if needed). I also see some very fine displacements on the left side, between some leaves. If that interacts with water (especially when it's reflective), I can imagine long render times.
You might even be able to fake water by using an 'underwater textured/colored' surface (flat), with RT reflection. That is, if you don't need to see rocks and leaves really 'disappearing' under the water level, if you get my point.