I don't know where this is, but I want to!

[Chimerix]

https://www.youtube.com/watch?v=Yz8GP_h22as

[Raptor90]

As one of the comments say: "This happened in South America in 2011.. Is Nahuel Huapi Lake, by side of Bariloche City, Argentina. It was after a volcano Puyehue erupted in Chile. Ash and very light and small rocks from volcano eruption covered part of the lake shores and stayed for several days floating on top of the water due it's light weight until sanks to the bottom. Today everything cleared and the water is clean as always was."

[Mynock]

If it's pumice floating on the water that's actually dangerous as hell to play around in. Be like falling through ice only the hole seals up behind you.

[Boggy Man]

Here are a few more videos:

Wind-driven waves in that stuff:

https://www.youtube.com/watch?v=_PY-AOowD_s

Someone scuba diving in it:

https://www.youtube.com/watch?v=SlYA9ZA-D9E

Someone rolling a rock onto it, with the rock sinking out of sight:

https://www.youtube.com/watch?v=tnbXQypO34Y

Someone kayaking through floating pumice and ash:

https://www.youtube.com/watch?v=xne4URl_MmU

Another video of waves moving through floating pumice and ash:

https://www.youtube.com/watch?v=HfcuwWiW4yA