The Diamond Beach is by Jökulsárlón glacial lagoon in South-East Iceland. The lagoon belongs to the Vatnajökull glacier national park. The glacier's name is Breiðamerkurjökull glacier and huge blocks of ice (more than 1000 years' old) are constantly breaking from it and large chunks of iceberg float in the lagoon.
The lagoon only appeared "recently", or around 1935, and has grown to ca 20 km2 due to the melting of Icelandic glaciers. It is the lowest point below sea-level in Iceland. And the deepest lake in Iceland, 248 meters deep.
The river used to run 1,5 km to the ocean before 1950, but now it is much closer and seals swim from the ocean up to the lagoon and rest on the icebergs there. The water in the lagoon is a mixture of fresh water and sea-water as the tides bring sea-water into the lagoon.