Once, I had the opportunity to walk on a floating mat on a guided tour with a nature organisation in De Wieden (Dutch: trilveen). On the surface, it looked (to my amateur eye) exactly like other peat, but walking on it felt like walking on a huge trampoline. Where I stood, I sank about half a metre down along with the moss, and the whole land in a radius of around 1–2 metre around me sank with me. When poking a long stick into the soil, we noticed that after a metre or so, resistance become zero: evidently, these mosses were floating on water.
Although rare, such floating mats exist throughout different regions in Northern Europe. I believe I have passed over one in Sarek National Park in Sweden, with no surface water in sight nearby but the entire pond covered I only realised it once I was in the middle of it (or at least I think it was a floating mat; I have no independent confirmation). I imagine falling through is an effective way to vanish without a trace, swallowed by the Earth with the land closing above me. I don't know if this is possible. The popular image of someone vanishing completely in quicksand is mostly wrong, as one doesn't normally sink beyond ones centre of mass. How are the real dangers for floating mats? Can one fall through? Can one get stuck? Can I pull myself out if I do? Can I pull out my partner safely?
In some parts of the world floating mats are only known to exist in access-controlled nature reserves, so anyone respecting the rules is very unlikely to accidentally walk on any; but that may not be the case everywhere.
The English language Wikipedia article states it is possible to "drown" in floating mats, but is unspecific on what this means: vanishing underneath the floating mat, or getting stuck in the floating mat.