The user in the video could not get Deus 2 to ground balance on the highly conductive sand. That could be operator error since there is no manual ground balance feature available in its Beach and Diving modes. It does have a ground grab and tracking. There is also a special Magnetic Conductive Ground-ground balance procedure but he did not seem to be using it or didn't know about it.
He changed from Beach (high frequency of 24 kHz) to Beach Sensitive (high of 40 kHz) several times. Some of the falsing was when he accidentally lost control of the coil. The Deus 2 manual repeats several times in its Beach section that keeping the coil level and the same distance from salt sand is important for quiet operation. I was interested in those magnetic/hot rock responses that may have happened because he could not get a proper ground balance.
An aside.....Homer in his Iliad/Odyssey spoke of the "wine dark sea". I could literally see dark red wine colored areas of silt? in that surf from the volcanic rock/sand. Really weird and beautiful.