I've found an interesting explanation regarding the origin and the adoption of the spelling of hammock. It is suggested that the term hamaca was misapplied by Columbus and his sailors which was intended for the native trees where the hammocks were hung, in the Caribbean (some sources suggest the Bahamas*, some suggest Haiti*, some suggest Fernandina island*) where they discovered it. These sleeping nets were used by the indigenous people called the Taíno and the Arawak. It is added that Bartram and his colleagues adapted the spelling hommock, then it changed to hammock.
Here is the relevant excerpt:
"Hammock, hommock, or hummock?" queried R. M. Harper in 1905; his answer was one of many attempts by botanists to reduce confusion over the classification of southeastern plant communities. Harper (1905) concluded that a hummock is a geographic feature, a rounded knoll, whereas "hammock" and "hommock" are synonyms for dense, hardwood forests that occur in limited areas amid the wet prairies, marshes, and pine forests of the coastal plain.
The etymoloogy of "hammock" and "hommock" is obscure (Oxford English Dictionary 1933). Columbus' sailors learned to use net "hammocks," hanging beds, from
Caribbean Indians; perhaps they misapplied the Arawak term, hamacas, intended for the woods in which the sleeping nets were hung. Bartram (1791) and other early expositors of Florida's natural history employed the "hommock" spelling, perhaps in keeping with native pronunciation, but "hammock" became the prevalent form in
the writings of the twentieth century.
The ecology of hydric hammocks: a community profile
Biological Report 85(7.26)
By: Susan W. Vince, Stephen R. Humphrey, and Robert W. Simons
Here is a relevant sketch I've found from the book Columbus and His First Voyage: A History in Documents (edited by James E. Wadsworth):
Additional good read about the word hammock and its origin:
'Hammock,' 'Hommock' or 'Hummock'?
Author(s): Roland M. Harper
Source: Science , Sep. 29, 1905, New Series, Vol. 22, No. 561 (Sep. 29, 1905), pp. 400-402
Published by: American Association for the Advancement of Science
Stable URL: https://www.jstor.org/stable/1633491