It's disappointing for the layperson because there's no basic explanation--I never thought of such a thing until 2:30pm Nov. 15, 2019.
Turns out NOAA does research into them and there's a whole organization dedicated to them. From that site:
WHAT IS A MARINE HEATWAVE?It seems that El Nino is a related phenomenon. And I assume that since the air and the water are both fluids, you could have some of the same sort of variations in temperature occurring in each.
We know that heatwaves occur in the atmosphere. We are all familiar with these extended periods of excessively hot weather. However, heatwaves can also occur in the ocean and these are known as marine heatwaves, or MHWs. These marine heatwaves, when ocean temperatures are extremely warm for an extended period of time can have significant impacts on marine ecosystems and industries. Marine heatwaves can occur in summer or winter - they are defined based on differences with expected temperatures for the location and time of year.