His requirements for the illustration were that the center of the image would be largely clear of details, as that would be where the lyrics for the specific song about The White Ship would go. This injected a fun design challenge to somehow make the illustration exciting while still keeping the details on the margins of the painting.
Here are initial thumbnails and designs:
I crafted the ship as being similar to a bird as in the story it is implied that the bird and the ship, as well as it's navigator/captain, are one and the same.
He eventually chose the top right one, mainly because it felt reminiscent of the cataract the ship eventual falls into, so I cleaned it up a bit and added the white bird (which I had forgotten) and this is what I had:
Behind the wave I originally had massive and mysterious structures (similar to what I had in the bottom left thumbnail in the thumbnail iterations above) but eventually dropped it as you really couldn't get a good idea of what it was and so would likely tangentially draw the viewer's eye away from the actual main story of the ship. I later replaced the structures with lightning to accentuate the chaos of the image.
I also dropped the sea-monster because it felt out of place in the composition of the image and, again, would take too much from the ship.
Here is what I eventually came up with:
It was a fun painting and I'm glad the client was happy the end result. And I hope to return to the land of The White Ship in the near future.