Giant Salvinia

Salvinia molesta
Giant salvinia is an aquatic fern native to Brazil.  Similar to Water Hyacinth and Water Lettuce, salvinia is a free-floating plant.  It reproduces asexually and can quickly colonize quiescent waters.   
This plant is a problem in many parts of the world and is considered one of worst weeds globally.  The socio-economic and ecological impacts of giant salvinia has led to its classification as a Noxious Aquatic Weed by the United States Department of Agriculture and the North Carolina Department of Agriculture and Consumer Services.
The negative impacts of giant salvinia include: degrading habitat (by blocking light and depleting oxygen), interfering/impeding recreation activities, and clogging irrigation systems.

 
The University of Florida Center for Aquatic and Invasive Plants has many excellent photos here:
https://plants.ifas.ufl.edu/plant-directory/salvinia-molesta/
Photo credit: Scott Robinson, Georgia Department of Natural Resources, Bugwood.org
Photo Credit:  R. Emens, NC Division of Water Resources