Well not quite, the vessel was a ten ton fishing boat but the jellyfish were damned large, if not out and out behemoths!
According to the Telegraph, the Diasan Shinsho-maru, capsized `as its crew was trying to haul in a net containing dozens of huge Nomura's jellyfish.

The jellyfish is one of the largest jellyfish in the world, growing up to two metres in diameter and weighing up to 200kg. Not much is known about Nomura's jellyfish, but its population can vary enormously each year. Last year had virtually no sightings of the jellyfish but this year the waters around Japan have been inundated with the creatures.
To end this sort of shaggy coelenterate story, The crew of the fishing boat was thrown into the sea when the vessel capsized, but they were rescued by another trawler, so all’s well that ends well...