This is the Silfra Fissure located in Thingvellir National Park in Iceland. This fissure is what’s known as a divergent boundary, or a rift valley created by the two tectonic plates between the North American and Eurasian continents moving apart. The plates are currently moving away from each other at about 2cm a year.
If you’re any sort of diver you need to add this to your list of places to visit. It’s water clarity is amazing (over 100 meters), as it’s mostly water from a nearby glacier Langjökull. As long as you don’t mind a temperature of about 2°C – 4°C all year round. The waters so clean it’s drinkable.
The National Park Thingvellir has been declared a world heritage site, and if you have family that aren’t into diving then they can go off on any one of the numerous walks in the area.