Keep in mind that while the stinging is not usually fatal to adults, it can be to tiny children when they encounter a massive school of adult sea nettles. Adults just wish they would die under these conditions. Back then, people would always have a gallon of vinegar on hand to rinse the affected area with and sooth the pain. When people were hospitalized, they were usually treated with antihistamines.
Ironically, I recall when the waters of Chesapeake Bay were pretty much free of sea nettles and other jellies most of the time. During the late 1940s, my parents would rent a car and drive to Chesapeake Beach, where gambling, mostly slots, was legal, the water was clean and clear, with underwater visibility to 15 feet during the summer, and the salinity was usually very high. Most of the time, when a sea nettle was encountered, someone would pick it up by the back, carry it to shore and place it in the sun to dry out. Most days we saw none at all.
It wasn't until the mid 1960s when upper bay beaches, such as Magavista, North Beach, North Point Park, and others began installing sea nettle nets to protect bathers from the jellies. A huge, downward trend in water quality and finfish populations coincided with the increase in sea nettles during the summer months. This is mainly because of what they feed upon, which is various forms of zooplanton, which thrives on other forms of planktons, which is the result of an overabundance of nutrients, from which the bay suffers. Fortunately, a big slug of freshwater and lower temperatures will keep them down the bay, but that same slug of runoff also adds to the nutrient load from agricultural runoff.
Keep in mind that if the good folks at Harrisburg didn't flush their toilets, Havre de Grace and Port Deposit, MD wouldn't have a source of drinking water.