In recent days, seawater near the tip of Florida has surpassed hot tub temperatures of 37.8C (100F), probably making it the hottest ever observed.
On Sunday and Monday, a buoy in Manatee Bay recorded triple-digit readings, while buoys in two other locations came close.
The highest ocean surface temperature ever recorded, albeit unofficially, was 37.6 degrees Celsius (99.7 degrees Fahrenheit) in Kuwait Bay in 2020.
Warming waters endanger marine life and ocean ecosystems such as coral reefs.
Scientists have detected bleaching and even death in some of the most robust reefs in the Florida Keys, which are home to millions of algae and other microscopic species.
When water temperatures rise above 35 degrees Celsius, corals expel algae, lose their brilliant color, and deteriorate. Such incidents can result in death.
Normal water temperatures for the area this time of year should be between 23 and 31 degrees, according to the National Oceanic and Atmospheric Administration (NOAA), which published the findings from the National Data Buoy Center.
Experts in the field warn that the increasing frequency and intensity of severe weather both on land and in the oceans is symptomatic of global, human-caused climate change that is fueling extremes, with current heatwaves predicted to last into August.
The World Meteorological Organization (WMO) reported earlier this month that worldwide sea temperatures have reached monthly record highs since May, owing in part to an El Nio phenomenon.
NOAA warned earlier this month that the warmer water around Florida could supercharge tropical storms and hurricanes, which build more energy over warmer waters. Rising temperatures are also severely stressing coral reefs, the agency said.
