Under Jolly Roger
Things fell into place quickly: I wanted ocean colours and white bamboo flakes (waves) as a base, and black silk strips to represent the pirate flag - and the skulls, of course, as they are also part of the flag, and therefore highly associated with pirates.
(sorry for the crappy pictures, I had to take it indoors, with artificial light and the colours are off)
I used more skull beads than I usually do, and I spun a highly textured art yarn from the batt shown above. It contains merino wools of dark blue and deep teal, hand dyed BFL/silk blend of a wonderfully intense dark turquoise, and white bamboo flakes. The black silk fabric strips make a great contrast. Here's a picture of the finished yarn in daylight, but since we don't get much light nowadays, it's still not as brilliant as it could be:
I still don't know whether I'll be able to part with this yarn, I am quite in love!
I also found another version of the song which I like a lot better than the one linked to above: Under Jolly Roger - by Therion.