The Silfra Crack can safely be counted among the world's top 5 most unique dive/snorkel sites. "Why," you ask? Well for starters, the "crack" refers to the fact that Silfra is the fissure between two of Earth's tectonic plates: the North American and the Eurasian. Thus this is the only place in the world where you can literally swim with one hand touching North America and one hand touching Europe/Asia. Secondly, the water, which consists of glacial melt filtered for decades through volcanic rock, is some of the clearest in the world, with visibility over 100 meters (yes, that's meters, not feet). This also brings us to unique point #3, which is that the water in Silfra is touted to be many times purer than bottled water. You can actually (and we did) dip your mouth in and take a gulp, which is a pretty incredible experience.
As for the diving/snorkeling itself, be prepared for dry suit conditions. The water is perpetually around 2-3 degrees C, so your hands and lips are going to eventually be numb no matter what. Secondly, unless you're a seasoned diver exploring the caves that lay deeper down in Silfra, the experience is fairly short lived. The crack only measures about 10 m deep in most spots and is not particularly long. That said, it really is an incredible experience, so get yourself hooked up with a local dive outfit (if you don't have your own gear handy) and go take advantage of an adventurous opportunity you'll only find in Iceland.