This 2014 video says about itself:
In this exciting adventure, Jonathan travels to Manuk, a tiny, uninhabited volcanic island several hundred miles from the nearest populated island in Indonesia, on a mission to discover why the waters of this remote place are teeming with thousands of venomous sea snakes!
From the University of Adelaide in Australia:
Sea snakes make record-setting deep dives
April 2, 2019
Sea snakes, best known from shallow tropical waters, have been recorded swimming at 250 metres in the deep-sea ‘twilight zone‘, smashing the previous diving record of 133 metres held by sea snakes.
Footage of a sea snake swimming at 245 metres deep, and another sea snake at 239 metres has been provided to University of Adelaide researchers by INPEX Australia, an exploration and production company operating in the Browse Basin off the Kimberley coast of Australia. Both snakes appeared to belong to the same species.
Sea snakes are found in tropical waters of the Indian and Pacific Oceans and are typically associated with shallow water habitats like coral reefs and river estuaries.
“Sea snakes were thought to only dive between a maximum of 50 to 100 metres because they need to regularly swim to the sea surface to breathe air, so we were very surprised to find them so deep”, says Dr Jenna Crowe-Riddell, lead author of the study and recent PhD graduate at the University of Adelaide’s School of Biological Sciences.
Oceanic depths between 200 and 1000 metres encompass the mesopelagic zone, sometimes called the ‘twilight zone’ because only a small amount of light reaches that depth.
“We have known for a long time that sea snakes can cope with diving sickness known as ‘the bends’ using gas exchange through their skin,” says Dr Crowe-Riddell. “But I never suspected that this ability allows sea snakes to dive to deep-sea habitats.”
These record-setting dives raise new questions about the ecology and biology of sea snakes.
“In some of the footage the snake is looking for food by poking its head into burrows in the sandy sea floor, but we don’t know what type of fish they’re eating or how they sense them in the dark,” she says.
The snakes were filmed in 2014 and 2017 using a remotely operated vehicle or ‘ROV’ undertaking work for the INPEX-operated Ichthys LNG Project. …
Published in the journal Austral Ecology, the study is a collaboration between the University of Adelaide, the INPEX-operated Ichthys LNG Project, James Cook University in Australia, and The Royal Danish Academy of Fine Arts (KADK) in Denmark.