I have watched Anne Stradal (The Cape Stitcher) do this with rapt attention for quite some time, and finally, with her encouragement, decided to try it myself - and I will tell you, it's addictive! She does hers on skies behind the lighthouses, but mine are intended for underwater things - coral reefs, etc.
I've studied many under water photos, and decided that my water needed to shade from light down to dark, and on a diagonal instead of straight down, as on the skies. This is a background for some sea fans, branch coral, whatever it strikes me to include when I'm done.
I just pulled out a 4" circle I already had drawn - stitch counted for symmetry, of course, and started stitching basketweave at the upper right. I decided in a hurry that it would take too long, and I'm impatient at this point, as I want to do the first sea fan with Memory Thread.
So - I'm stitching over two stitches instead of making a single "tent" stitch, but in a basketweave format. This probably has a very grand name in a book somewhere, but I'm unaware of it - and I was actually rather pleased that it looks kind of ripply. Having pulled out a skein each of 597 and 598, I started - and was amazed at how the shading progresses. Awesome!.
I don't want to go in to the particulars about the needle blending (I've decided to name mine "thread shading.") as you can go to Anne's blog and look at hers - she explains it very clearly.
When I finished with the darker shade, I still had about half of the water area to fill, and decided to try adding even more - and get darker, but a little bit green. Hopefully, I have calculated (mentally) correctly, and will have room to end with the very dark greenish teal. I'll show it tomorrow if I get it finished.
The skein of 712 is for sand, which will be a background for some coral and maybe a tiny sea shell.
Incidentally, this is how an idea becomes a design sometimes. I've seen this in my head for several months, and finally decided it needs to be done. If it's a mess, I'll just throw it away, sulk for a while, and do something else.