From the article:
“One of the more fundamental reasons is that the ocean is warmer now and warmer water holds less oxygen,” says Chan. “And then the second part is that a warmer surface ocean, it acts as an insulating blanket.”
So that blanket stops colder low-oxygen water from rising up and mixing with oxygen in the surf.
Scientists say climate change is behind this. The ocean has been absorbing nearly all the rising heat from greenhouse gas emissions, and it’s projected to grow even warmer in coming decades.