Jellyfish are the very definition of spinelessness. Because, you know, they have no bones and just seem to float wherever the ocean takes them. So it's kind of weird that in a novel about dreams, the very first thing we see is a jellyfish.
Well, if you really think about it, it's not so weird. Jellyfish are kind of like sleepwalkers: they move in all kinds of directions without being able to control it, and they don't even know where they are. Sounds like symbolism to us.
Let's dig a little deeper, starting with the first line of the novel:
Current-borne, wave-flung, tugged hugely by the whole might of ocean, the jellyfish drifts in the tidal abyss. The light shines through it, and the dark enters it. Borne, flung, tugged from anywhere to anywhere, for in the deep sea there is no compass but nearer and farther, higher and lower, the jellyfish hangs and sways; pulses move slight and quick within it, as the vast diurnal pulses beat in the moondriven sea. Hanging, swaying, pulsing, the most vulnerable and insubstantial creature, it has for its defense the violence and power of the whole ocean, to which it has entrusted its being, its going, and its will. (1.1)
Perhaps the most important part of this passage is the last sentence. Even though the jellyfish seems vulnerable and defenseless to us, Le Guin points out that its strength and defense is the power of the whole ocean—and that's a lot of power.
George is frequently compared to a jellyfish. Just as the jellyfish seems to be defenseless, George seems to be a wuss for most of the novel. He just moves along, not really taking action or forcing his will on anyone—but, just like the jellyfish, that is exactly what makes him strong. That is why he's the only person who is able to save the world when Dr. Haber starts dreaming.
Think about it. The jellyfish doesn't rely only on its own strength; it has the strength of the ocean to protect it. Well, George has the strength of the whole universe helping him. He's not alone, and he's only able to do anything because his friends help him out.