Oh, and did we mention that one of the species exploding in population is the box jellyfish, a variety previously found in the waters off northern Australia? And that many of these things possess 6-8 foot long tentacles covered in venom that can kill a human in three minutes?.
What the hell did we do?
Three things: First, we're dumping massive amounts of agricultural waste into the ocean. The fertilizers present in that waste are designed to increase plant growth, but they'll do their job on algae just fine instead. That algae, in turn, feeds microzooplankton, which, along with a glass of orange juice and an inspirational speech from Tony the Tiger, are a big part of any jellyfish's balanced breakfast.
One day they'll learn to fly, and mankind will reap a just harvest of pain.
Second, the ocean in general has gotten warmer. Whether you want to chalk that up to global warming or angry wizards, the temperature has risen recently, and for unknown reasons that makes jellyfish both reproduce more and swim closer to the beaches. This particularly applies to tropical jellyfish -- you know, like that three-minute-death tentacle kind from earlier? That's what's waiting for you out in those clear blue waters, just humping and murdering up a storm, waiting for your children to swim on out and join the poison-orgy.
It's...less fun than it sounds.