Thanks to all who have responded to my original query about Indian Ocean piracy.

Allusions to "pirates" and "piracy" permeate Mark Twain's writings. His
"Salutation from the 19th to the 20th Century" is an example:

"I bring you this stately matron named Christendom, returning bedraggled,
besmirched, and dishonored from pirate raids in Kiao-Chow, Manchuria, South
Africa, and the Phillipines, with her soul full of meanness, her pocket full
of boodle, and her mouth full of pious hypocrisies. Give her soap and a
towel, but hide the looking-glass.

"Mark Twain
"New York, Dec. 31, 1900"

This interesting comment appears in a notebook entry from 1888 or 1889: "A
monarch is perpetuated piracy. In its escutcheon should always be quartered
the skull & cross-bones" (N&J 3:401).  A similar allusion to monarchy and
the skull and crossbones appears in chapter 10 of THE AMERICAN CLAIMANT. Tom
Sawyer imagines himself a pirate flying a skull-and-crossbones flag in
chapter 8 of TOM SAWYER, and in chapter 39 of HUCKLEBERRY FINN, he draws a
skull and crossbones on the door of a house.

So, for what it's worth, Mark Twain associated the skull and crossbones with
piracy at least as early 1875, when he was writing TOM SAWYER, and probably
much earlier.