Thanks Mick
Yes, the water's edge (sigh), it's a pain! Technically it's correct. In other words; the water meets the terrain everywhere in this image, so the plane isn't being cut off somewhere.
The straight look is just because the slope isn't very steep.
The transparency is something I can't get right somehow.
At the moment I'm working on improving it, but the refractions look so fake. I wonder if they actually are really volumetric. At these angles it looks like a fake volumetric effect, just as the volume density shader (which as a matter of fact IS a fake volumetric effect).
Another problem is having the water perfectly transparent at the edges, despite having transparency @ 1 and transparency color pure white (no density used).
Another problem is that under water shadows have no GI.
There's a lot to improve to make this one really work.