Although relatively little known, there are a few sightings that suggest the possibility of gargantuan jellyfish somewhere in the ocean.
In 1973, the ship, Kuranda, collided with a gigantic jellyfish while sailing to the Fiji Islands. One seaman who was hit by the jellyfish's stinging tentacles came away with severely burned skin (he died from the injury). The jellyfish also started to push the ship down with it's weight. An SOS resulted in help from the Hercules. The crew of the Hercules managed to get the jellyfish off the ship with a high pressure hose. Later, when the slime from the jellyfish was analyzed it turned out to be from a species called lion's mane.
In the mid-1950s, a diver, while watching a shark, saw a big, brown shapeless thing head over towards the shark. The shark convulsed, and then didn't even try to escape as the thing came and grabbed the shark. Chances are that a large jellyfish would best fit the description.