We see these often on the beaches from Campbell River to White Rock; usually by the time we get to them, they're little more than a mass of red goop, about the size of a flat soup bowl. This one was probably dead, too. It wasn't moving, except as the gentle current moved the tangle of seaweed beside it.
I think it is floating upside-down. The bits of torn rope in the centre are probably all that remains of the mane; possibly those delicate white threads around it are some of the fine tentacles. I didn't dare try to fish it out; dead or alive, these critters sting.
"The stinging cells ( nematocysts ) in the tentacles can actually live for months after they are detached from the Jellyfish." (Emerald Sea Photography)
The Lion's Mane is a cold-water jellyfish; it is "seldom found farther south than 42°N latitude," more or less on the Oregon/California border. (Wikipedia) The colder the water, the larger they grow. They live for one year; towards the end of the summer, they gravitate towards the shore, where we find them cast up, melting.
Emerald Sea Photography has beautiful photos of the live jellyfish.
North Island Explorer has a photo showing those translucent lobes curled up around the rim of the red centre at the top.
And here's one with the white bell stretched out, swimming. This one, found south of Seattle, was 14 inches across, a bit bigger than "ours". The tentacles reached 8 feet down.