Yes, the Garmin Edge 530 is waterproof. The device is rated as IPX7, which means it has been tested for immersion in 1m of water for at least 30 minutes, and remains waterproof.
Given that no-one rides a bike with the handlebars submerged that far under water (well, mainly no-one), the Edge 530 can be classed as fully waterproof from a cycling perspective.
(And, for completeness, you don’t need to buy a separate waterproof case for the device).
Here is the evidence of my unscientific test:
And thankfully, here is the result:
And it still works afterwards (as it has been built to do):
Now you know it’s waterproof, the links are below in case you want to buy one.
Also, check out my post comparing the Edge 530 with the
Compact bike GPS offering high performance at a reasonable price. Intelligent route mapping, unlike the previous 520 model. Not touchscreen (but this isn't a dealbreaker for me).
Here is the video of me dropping an Edge 530 in a glass of water: