OK, a couple final observations:

If you do want to use the iso rather than a heightfield, I noticed that your
isosurface is clipped too high and intersects with the floor, this will get
noticeable when the light fading is working properly.  Change your contained_by
box (This is likely also increasing your render time unecessarily)
   contained_by {box {<-10,-3,-10>, < 25, 1, 25>}}


If you are not happy with the way the lightfading is working switch to media,
with scattering only it doesn't add much render time: (be sure to add "hollow"
to your isosurface or heightfield object for it to work)

#declare WCol=rgb <0.217,0.552,0.877>;
#local FD=3;
#declare i_water =interior {
    ior 1.33
      media{
        absorption rgb 1-WCol
        density{rgb -ln(0.58)/FD}
      }
}


I did some very quick time comparisons for you:

1) Original iso with fading: 56s
2) Corrected iso (as above) with fading: 40s  (big increase! doesn't have to
calculate back intersections anymore
3) Corrected iso with media: 42s (only slight increase)
4) HF (600x600) with fading: 28s  (another big decrease!)
5) HF (600x600) with media: 30s  (again, only slight increase)

Also note that changin the resoltuion of the HF only increases parse time (and
memory usage) not render time.

Hope this has all been of some help to you.

-tgq

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