Had no idea what this stuff was. Never would have guessed that it was a fern! Carolina Mosquitofern, an aquatic species with much-reduced fronds. Sorry for the poor photo quality; zoom shot from a bridge far above. Also present are duckweed (larger green plants; Lemna sp.?) and watermeal (tiny green; Wolffia sp.).

Azolla forms a symbiotic relationship with a nitrogen-fixing cyanobacterium, Anabaena azollae. Since that lets it obtain all the nitrogen it needs, the only limitation on its growth is phosphorus. Extensive blooms like this may thus be a sign of eutrophication from fertilizer runoff.

Here's what Weakley says:
1 Phyllaries appressed; rays usually < 20 [(10-) 12-23 (-34)]; [section Heterophylli].
2 Middle stem with a winged, sheathing petiole; involucre (4.2-) 4.5-8 mm high; disc corollas (15-) 19-33 (-43); phyllaries acute, green blaze on phyllary diamond-shaped, about as long as wide or slightly longer.
3 Leaf faces glabrous.
4 Leaves cauline, the largest on the stem; largest leaves narrowly to broadly lanceolate, avg. < 9× as long as wide; leaf margins usually only slightly scabrous; [mainly of inland provinces, of NS west to MB, south to GA, Panhandle FL, MS, LA, and OK].
5 Larger leaves > 5× as long as wide, rarely > 2.5 cm wide, the bases slightly clasping; [NYand KY south to GA, Panhandle FL (Jackson County), and MS] ............................................................................................................................................... S. concinnum