Along with this awesome sea lion photobomb pic, a jellywatch.org user reports over 100 sea nettles in Monterey, California on Friday, saying:
Sea nettles are back in the Monterey marina in large numbers. Usually there are all sizes from tiny to large, but this time they are mostly large
Be careful swimming folks, these guys can pack a sting. Pee also won’t likely help much, try lidocaine to neutralizing stinging cells and for pain relief.
The Helm Lab Blog 
I’ve seen these washed up on the beach in Oregon, USA, frequently, but never tried to learn what they are. Now you’ve got me interested. And jellyfish watch sounds promising too. Thanks!
Reblogged this on Bay-to-Beach Life and commented:
What a picture! Sea nettles may be tough on the sea lions, but it’s great kayaking over these swarms.
Oh awesome– I’d love to do that!