When it's algae and it's actually blue-green and it's been found in Pigeon Lake in Peterborough County, Ontario, Canada.
Why is that bad? Blue-green algae, also known as cyanobacteria because it's not really algae, can produce cyanotoxins into the water when harmed. These can cause all sorts of health problems up to and including Lou Gehrig's Disease. One 2005 study looked into cyanotoxins' potential military applications. We could run down a list of other potential symptoms, but those two tidbits ought to get the message across on their own: if blue-green algae is in the water, you're not if you know what's good for you.
There's also an advisory for Ohio's Buckeye Lake in Fairfield County, and it has shut down two ponds in Massachusetts and Meade State Lake in Kansas, among many others. The Great Lakes in particular have seen an explosion in blue-green algae blooms. Why? Long story short, zebra mussels, a type of mollusk that devour so much plankton and reproduce so quickly that they choke out the food supply of other fish. In the process, blue-green algae, which the zebra mussels aren't interested in, has a chance to grow unchecked.
Zebra mussels are to the Great Lakes what rabbits are to Australia: neither is supposed to be there, and in the minds of the local experts, the only good specimen is a dead one. The rabbits will eat your garden. The mussels will eat your boat, or at least clog the motor's intake valve. They'll also stick to the hull, and if not removed before the boat is used on another body of water, that body of water, if it didn't have a zebra mussel problem before, will have one now.
There's also the quagga mussel, which experts think is even worse.
The process for eradicating a land-based invasive species is long, incredibly difficult, and rarely if ever succeeds. The process for eradicating a water-based invasive species is 'yeah, good luck with that'.
You may want to stick to waterparks this year.