Update 12th July 2020

Fish Scales
When I tried this a few years ago, it didn’t work, but this time it did.  Main difference between mindsets was: former had been pursued as per surface ‘embroidery’, e.g. decorative, flimsy, unstable.
Latter approached as mini ‘weaving’, hence stable, firm and consistent results.

So that’s the mermaids’s scales sorted.
In researching how to approach the fish scales, I found this reproduction of a ‘boiled leather cuirass’ from 9th century soldiers, worn over chain mail.
Later this design pops up in falconry gloves.

Medieval Collectibles

Now onto the Mermaid’s German headgear, which I managed to confirm this morning

Respect due to the model here, her hair AND headgear is exactly what I can see in this woodcut.
Which is great for historical accuracy but not so great for overall design....you’ll see what I mean later.  Then again, I think if her head gear is white it could tie in with the sails?

Then there’s the mermaid’s tail background layer.
I’m going with this idea, I’m about to do a couple of try outs.

This is two shades of woven kid, soft leather.  Now I don’t have soft leather, so I’ll use something else.  This method will save a lot of work, because the sea is taking such a long time....

I created a firm base for her tail with reversed chain stitch. That stitch takes such a long time.
I also have to switch to stem stitch stab method for much of the sea, again that takes forever, instead of the sew method, which is so fast and fun.
At long last I was able to photograph the piece in natural light. The second fish is not a shark.