There is a Marshall V. Simmons listed in a 1849 New Bedford, Massachuetts city directory, who is a shipwright. Since first names run in families you might try posting to the MA message boards for Simmons from MA who migrated to GA.
This MV Simmons in MA is also listed as a lighthouse keeper of the Bird Island Lighthouse in MA. Google "Marshall V Simmons".

Have you looked at other Simmmons family living and listed on the same censuses as Marshall/Marshal? Looks like he has siblings Cornelius S and Abraham. (1870 they are listed as Simons as are the last 4 of Marshall's children on same census page, Marshall and rest of family on last 6 lines on previous page.

According to Marshall's wife, Margaret, her sworn statement filing for a FL Confederate Widow's pension she stated Marshall died Sept 6, 1906 in Gladsden County. There was a State Hospital located there. The FL online database has the V transcribed as an N so the index is listed Marshall N. Simmons, Margaret E. Simmons, widow. She also states except for 3 yrs living in GA, they lived all thier married life in FL. Daughter Susan age 18 in 1870 census has b. GA.

Oldest son is named William. There is a Willima M. Simmons b. abt 1790 MA listed in 1850 GA Bibb Co. census, no additional Simmons family in household. He is a Physican.

For other Simmons researchers Marshall V. and family listed after him and before Elizabeth A Simmons in 1860 FL census is listed as "Sim" and in Hamilton County for Ancestry's index, the wriiten page is Hillsborough County.