I hesitated to weigh in on this discussion, given the Twain credentials of  
the paraticipants so far. But I will.
 
I once worked for a Governor, who commented that "if you want to repeal a  
law, consider why that law was passed in the first place." I applied that  
approach to the question of Twain's selection of location for, and use at  
all of, "Pirates in the Indian Ocean" when Huckleberry Finn was dictating his  
life story in the early 1880s.
 
History records no piracy in the Indian Ocean during the 1860's-'80s.  
However, the opening of the Suez Canal in 1869 reduced the volume of shipping  
passing through that ocean, and in the process brought the name into American 
 newspapers with a fair regularity in the 1870s. Great Britain maintained 
its  hegemony over the Indian Ocean, and it was considered safe passage, but 
longer  than the Suez. Maritime insurance did not consider a ship "lost" in 
the Indian  Ocean unless it was not heard from for nine months. On other 
oceans ships were  lost after six months. Piracy was occurring in the Hong Kong 
area when Twain was  writing Huck, but it was not a big deal in newspapers. 
The Ilanun of Borneo, the  most feared pirates in  the waters around 
Southeast Asia (well east of the Indian Ocean, which  is south and west of India) 
during the mid 1800s, wore sarongs and  embroidered belts. Little is seen in 
American newspapers of that  gang. 
 
The Pirates of Penzance opened in New York City in 1879, and was a  big hit 
for years after that, performed all over the nation. Wikipedia says over  a 
hundred companies were soon performing it, unauthorized, in America. I 
don't  think Pirates of Penzance is set in any specific ocean, but it brought  
the word and concept of Piracy into a front row seat. 
 
In the November, 1870, Atlantic Monthly, Harriett Beecher Stowe provided a  
long, colorful tale of Captain Kidd, who died in 1701. In 1880 an article 
about  men seeking to locate Kidd's gold on the East Coast was widely 
reprinted.
 
Finally, consider the mental image of "pirates in the Indian  Ocean" versus 
"pirates on the Atlantic" Indian Ocean presents a much more  dramatic 
cachet 
 
Bob Stewart