I've filled in most of the black areas by hand and then scanned the result into the computer. I am unsure how to approach the two white areas: the red algae flower in the upper right and the coral in the lower left. Here is the scanned version. I have printed this out and will fool around with shading those areas. When I have achieved the look I want, I will copy it onto the original before scanning that final into the computer. Then I will inverse the image to see how the black on white version looks.
So, here is step 2.