i have been feet first vertically so deep in a peat bog that i could barely break the surface with my arms upstretched above my head, im very much into total submersion and being able to stay below for a while and enjoy,. so i use a simple length of hose as a snorkel,. inhaling through the hose and exhaling through my nose so that i dont rebreathe the air in the hose,. as bogbud has said the limiting factor is the bouyancy,.so ive only been able to reach these sort of depths in organic peat bog type mud which is pretty low density and even then it is very hard work sucking air through a hose at that depth due to the pressure of the mud on your chest, ive never been able to get more than 6 inches under in heavy sediment type mud and its not really sinking,. more yanking yourself under as its like trying to sink a buoy under the water, i just bob back up to the surface because the mud is denser than i am.
i built up slowly when it came to submersion and remember feeling a mixture of excitement and terror on those few occasions where i had started to pull myself up and realised i wasn't surfacing as fast as i would have liked and might be stuck below the surface,.. the only times ive been stuck is of my own making,. i used a few tricks like wearing heavy inflexible boots or even a backpack with weights in it for those moments when i wanted to feel truly stuck,. however i was never really in any danger as i always made sure it was easy for me to slip out of my pack or boots when i was ready to escape
i also really enjoy jumping feet first into a bog feet pointed down with arms by my side just to see how far down i will go, and ive also enjoyed many head first dives with an aim to vanish completely headfirst (although my feet have almost always still been above the surface) not recommended in a bog you don't know well or if your not a strong swimmer with plenty of deep sinking practice
those days are pretty much gone for me now as ive moved so far away from any viable sinking spot and spend my life surrounded by city and people,. one thing i do enjoy is being on communities like this and enjoying seeing other peoples adventures to remind me of how much fun you can have in a bog you know well