This dive is between two tectonic plates in Iceland. It’s quite shallow (less than 60 feet deep). But the water’s cold – 38 degrees.
“The SILFRA Rift is an absolute world class dive site due to two main reasons. First of all you will be diving in a crack between the American and Eurasian continents. It's the place where the continental plates meet and drift apart about 2cm per year. Secondly the visibility that you will experience will rarely be surpassed, if ever. 100m+! The reasons for this clarity are twofold: the water is cold ( 2°C - 4°C all year ) since it's the melting water from a glacier about 50km away and has traveled through the lava fields for many years before coming out at the north end of Thingvellir lake through underground wells.”
A youtube video is here.