As I said I never used David's software and have no clue.
Appatently you must know what is land and what is sea and
then apply line one or line two of your LUT. This of course
cannot be done by looking at a single VIS channel. As your
image shows nice coastlines as well I suspect there is
something like a bitmap applied which knows what is land
and water. This of course is not how I use a LUT with IM.
What software do you make yout LUT with? I had a closer
look at your *.bmp and AFAIK it has a problem in the 54
byte BMP-header. There is a "uint32_t image_size_bytes"
field in the header that I expect to be 1590-54 = 1536
in the case of your uncompressed two line LUT. But with
my homebrew BMP-reader I get "image_size_bytes = 0" !?