At least in the context of the fragment I posted, it seems certain CFS was refering to Rosario, Argentina.

I went through my collection of CFS poetry books again last night, and other than being diverted for several hours I turned up no relevant clues.

I may do better if I can gain access to the Joanne Colcord archives here in Maine at the Penobscot Maritime Museum. The poem may be in a private letter from CFS to Colcord. The poem fragment was actually cited, pp. 16-17, in a recently published collection of Colcord family letters entitled LETTERS FROM SEA 1882-1901, edited by Parker Bishop Albee, Jr., Tilbury House, Gardener, ME, 1999.

Cheerily,
Charley Noble