What we do have is some light to moderate Sargassum seaweed, sometimes in large rafts floating offshore.  I think that's a sign of a healthy ocean (even though it makes fishing, swimming, and surfing unpleasant).  I have a white boogie board and I'm always shaking the seaweed onto the board so the kids can see the small crabs, shrimp, and fish that life in the weed itself.  (And note that Sargassum is an effective capture and storage pathway for CO2.)

Other parts of the Gulf are much degraded, such as the "dead zone" that floats like a ribbon from the Mississippii, and red tides of Florida, and the giant cabbage head jellyfish near Corpus Christi.  So I'm not disputing the article, other than to point out a great deal of spatial variation.  Second there is a whole bunch of temporal variation because of occassionally increasing "blooms".

A troublesome aspect are persistent jellyfish blooms, such as notable one in the Med.  The Portuguese Man-O-War sightings in lower New England were a function of current and storms, however.  But I did notice a profound change in jellyfish blooms after a large nuclear facility (Millstone) was built in Connecticut in the 70s, and the Red Mane Jellyfish seemed to bloom all summer long, like several per square meter of water!  This had never happened before, and the Red Mane was more associated with cooler waters, NOT warming waters.  That was very strange, I will agree, although I have not witnesses any such blooms recently - just continuting hypoxia in the western Long Island Sound, and a general die-off of nearly all the lobsters in southern New England.  Correspondingly, there has been reports of a dramatic reduction in Comb Jellies (Ctenophores).

On Ocean Day is it important to note how things work together, like the decline in the sand dollar species, coral reef degradation and bleaching, starfish populations, and invasive species such as the Lion Fish.  There does seem to be a northward encroachment of some tropical species, although some are cyclical such as the Humbolt Squid on the West Coast, due to El Nino / La Nina (ocean ascillation) events.  Interestingly, some of these invasives are bacterial level, such as Vibrio Vulnificus, a more tropical and potentially deadly strain (you could die from a cut that got infected, or have neurologic disorders forever).  But I do have to admit, jellyfish are pretty cool animals.