The best dives of Iceland are:

Silfra is one of Iceland’s most famous divespots. It’s a rift that is part of the divergent tectonic boundary between the North American and Eurasian plates. It contains clear, cold water that attracts scuba divers from all over the world because of its high visibility and geological importance: you literally swim between continents. The rift offers amazing visibility and has been placed on the world’s top 50 diving destinations. The visibility reaches end-of-sight and is rated at 150 to 300 meters. Some people say that it feels like flying. Reykjavik is a good base for taking a trip to Silfra.

In the north you can dive on/at the Arctic Circle and at Strytan, a giant submarine cone, releasing about 100 liters of hot water per second. Normally, you only find similar type of cones at a depth of at least 2,000m deep, so this is truly a unique phenomenon and the only one in the world.