However, an anonymous commenter (see Comments) pointed out that these plants were probably Helianthus simulans, common name muck sunflower. Although their leaves are narrow, they are wider than H. angustifolius. After more investigation and measuring leaf widths, I concluded that the commenter was correct. The sunflowers shown here are H. simulans.
The USF Plant Atlas characterizes H. simulans as "Not Native." Wunderlin states it is native to Louisiana, but has escaped cultivation and is now found throughout the southeast.
Swamp sunflower; narrowleaf sunflower (Helianthus angustifolius, Asteraceae)