This gorgeous footage is of Sarosh Georges Jacob taking a swim* in Lake Palau on Eli Malk Island in the Republic of Palau.The lake, commonly called Jellyfish Lake, is packed with beautiful jellyfish.
According to the video description about 12,000 years ago some jellyfish got trapped in a natural basin on the island when the ocean receded. Living in peace and with no natural predators over the thousands of years the jellyfish populated the lake and lost almost all of their stinging ability. They are essentially harmless to humans except if you have very sensitive skin and then you might feel a bit of discomfort when the brush against you.
There are more details about the jellyfish in the original video description that are well worth a read. Go visit if you have a chance.
(Actually to be fair it is not entirely clear if that is Sarosh himself taking the dip or if he is just the editor/filmmaker. But in my world I have decided that is he because I like it that way:-) )