Adrian,
I think the water colour is fine (see Strats second image), just a bit more life in the dispalcement/bump. Also, you mention refraction is on because you don't want to see the pillars meet the sandbed, but isn't that what you would/should see? At the moment the post look like they are sitting on the water, not in it.
Apart from that and the pool overflows (that just look too solid but if animated might look OK) I think it's looking great. Looks like a great project and you've put a great deal of effort into it.