I'm not a girl (even though I almost always wear a skirt, drifting off topic a bit but they are just so much nicer to wear and I hate it that it's not normal for men to wear skirts) but I have sunk completely under many times.
I always sink fully clothed, that is part of it for me, getting clothes completely covered in mud, and the fantasy that I had just unexpectedly come across the mud and fallen in with no preparation.
A while back I had found an area (now sadly built on, by what looks like an Amazon fulfilment center) with a large, deep area of silt. It varied in quality depending on how much it had rained, but it was best when it was reasonably thick (and this also made it easier to go under). When I stepped in, I very quickly went in up to my waist. The mud felt very smooth and satisfying on my thighs. Then as I would get deeper, the feeling would change as the mud pressed my shirt against my body, a feeling which I love for some reason, along with the feeling of the wet mud sliding up along my thighs. One thing you note is that you become extremely buoyant when about chest deep, and won't sink any deeper without something to anchor yourself on. I tend to avoid footwear as the few times I've tried with shoes on I've always regretted it (including almost getting stuck once). At least in my old mud spot, the deeper mud was a bit thicker, and there was usually a thicker layer I could jam my feet in and pull myself further in. It took quite a lot of effort, it was always a good aerobic workout, but eventually I'd get neck deep and then slowly pull myself under, tilting my head back as I went down to get "one last desperate breath". The odd thing is the mud always feels gritty when going over the face, I think it's just that the face has much more sensitive nerves than the legs. I would then hold myself under as long as I could. Once out of breath I'd just relax and float back up to the surface. Usually at that point the sensations would get too much and the orgasm would be explosively good, full body, with waves of sensation.
The trouble then was the post-nut clarity
here I would be completely covered in mud, and having to get home without getting mud over everything. After the first couple of sinks I had a full preparation routine, the trouble was at this mud spot there was very little way of clearing up afterwards so I'd always still go home a bit muddy regardless.
I have managed to go sinking a couple if times, but only managed thigh deep, which was amazing. I've always wondered what it must feel like to go completely under, and would love to hear the thoughts of any of the girls who contribute so much for our enjoyment. 
