Found the culprit; seems that my gradient wasn't exactly zero on shore, so a lot of quite invisible reflectuions had to be calculated on rough ground. Added a color adjust where I raised the black a bit, and hopsa, a faster render.
In another work on waves there seems to be a hard line in the hollow bit, but it's to do with the reflective shader.
And for those of you wanting to try the blue nodes... a little of my node setup