The title of my forthcoming book, From Swamp to Wetland, references the changing ways that human perceived the Everglades and other wetlands in the twentieth century. Swamps were vilified landscapes. The Everglades was a miasmatic wasteland, the haunt of noxious vermin and poisonous snakes. Of course a swamp is technically a tree-dominated wetland – a marsh is a plant-dominated wetland – but Americans often used this term as an epithet, not in its biological-accurate usage.
Beginning in the 1930s, inspired by ecological and biocentric ideas, Everglades National Park advocates redefined the Everglades. Now it was a tropical, biological wonderland, defined by its myriad expressions of water. Open bays, shallow seas, estuarine labyrinths, sandy beaches, and glistening marsh defined the Everglades. The Everglades was the habitat of fantastic flora and fauna, including wading birds, alligators, manatee, palms, and orchids. The Everglades is now a wetland.
As you can see below, popular usage of the terms swamp and wetland likewise changed over time, reflecting the rise of ecological ideas and the reevaluation of non-traditional landscapes like wetlands, deserts, and jungles.
The following chart is a Google Ngram, which charts the frequency of these terms in google’s library of digitized texts.

See this image in more detail here.
The changing usage of these terms can also be seen through an analysis of selected Florida newspapers. The charts below show the number of pages on which these terms appear in the Tampa Bay Times, the Fort Myers News-Press, the Palm Beach Post, the Fort Lauderdale News, and the Miami News. This data was taken from newspapers.com, where all these papers are digitized.
The usage of these terms is quite consistent, except for the Miami Daily News, which is an outlier in many ways. If we look at the first four papers, some common themes emerge.
As you can see in these charts, use of the term wetland can first be seen in the late 1950s, and becomes more widely used by the late 1960s. The term is in wide-spread use by the 1980s, and in three of these papers, it even eclipses the use of the word swamp, which is astounding considering the fact that many proper place names in Florida include the word swamp and the fact that a swamp is also a widely-used term to describe tree-dominated wetlands. The word swamp may have lost much of its negative connotation in the second half of the twentieth century as well, and it is possible that newspaper editors opted for the term wetland because it applies to a wider variety of landscapes. Of final interest is the fact that for the three papers that still currently publish, use of both terms sharply drops off right around 2009. This suggests that perhaps these papers all sharply curtailed their environmental coverage during those years.