In reality, it's impossible to drown in quicksand unless one goes into it head-first, as its density is roughly twice as high as that of the human body. The danger comes from the adherence of the muddy substance, as well as the suction effect, which makes getting out of it unaided difficult and exhausting. However, as (sort of) shown in the episode, one of the biggest risks is pulling in others who are trying to get the person out. Of course, in Archer's comatose dream, anything is possible.