Diving and snorkeling may not be the first activities that come to mind when you are travelling to Iceland. It is quite understandable, Iceland is a cold country that is filled with volcanoes and most people usually go diving in warm tropical places? The fact you are forgetting of course is that Iceland is also situated ontop of a techtonic rift.
Diving and snorkelling is becoming an increasingly popular activity for people who are travelling to Iceland thanks to the silfra fissure. Silfra is the name given to the fissure that opens up in Þingvellir national park, which seperates the American and Eurasian techtonic plates. The water that fills this fissure is extremely clear, offering visability in the range of 70-80 meters. Perhaps more importantly it is only one of two eassily accessable locatoins in the world where you can swim in these conditions.
The water may be cold, but the sights are more than worth it and thanks to the insulated dry suit you will usually be issued with you probably won't notice the cold anyway!