The Nauvoo Neighbor of 17 September 1845 lists the "Sexton's Weekly Report" on p 3, and listed in there is "Wm. Williams, 20y Billious fever." He is not listed on the Nauvoo Sexton's record has been published (go to http://mormonhistoricsitesfoundation.org/publications/studie... to download a pdf copy), but he is not listed as being buried in Nauvoo. That does not mean that he was not, as there were many family burials that the sexton did not record.

There was an 1843 Hancock County map made, that was published in Ida Blum's "Nauvoo: Gateway to the West," and it shows several islands in the Mississippi River around Nauvoo, including two large islands immediately south of Fort Madison, IA. Unfortunately, it does not identify any of the islands by name, and modern maps do not show these islands because when the dam was built at Keokuk/Hamilton in the 1910s, it raised the level of the river, and these islands were submerged. At least, on the map they were. Portions of them still exist, and are visible from the Fort Madison/Niota bridge. There were also other islands, two of them were between Nauvoo and Montrose, IA, but also unnamed on the 1843 map.

If you REALLY want to know which island was Madison Island, you could try contacting the Coast Guard and asking if they would check their pre-1920s maps to see if they list it.