All the criteria was met with the above illustration but I felt it wasn't good enough. The sea wasn't particularly well painted and the Merman was flat along with the submerged sharks. I was trying to pull back on detail so I put this failure down to my lack of skill working loosely with watercolour.
After some time of pondering my finished illustration I decided to do another version that was more in my comfort zone. I deliberately concentrated on the shape and form of the Merman and sharks which led to my decision to simplify the sky. I also stylised the sea so that underwater details were easy to see and hopefully didn't complicate the image.
I was much happier with my second version and so was the Folio Society but in the end I still didn't get the seal of approval. Being an illustrator you get used to rejection, it may be depressing on the day you hear but by the next day you have to move on and learn from your mistakes and above all improve.