Here is a technique that I tried and it worked perfectly. I was kayaking on a large body of water, secluded on all sides by tall bulrush plants and only a foot or less of water on top of a thick under-layer of fabulous mud. I wanted to play in that mud, but without the water being on top. So I made it happen and here is how you can do the same thing.
I chose a spot to create my mud area and went outside of this area and dug the mud from the bottom and built a sort of dyke or wall in the shape of a large oval, about 10 feet long and 7 wide. The pond now had an oval-shaped pool of pond water surrounded by a dyke. The mud was still beneath nearly a foot of water inside and outside of the enclosed area.
So next, I took my kayak paddle and used it as a shovel, scooping out more of the mud below and tossing it into the middle of the enclosed area. Eventually, the enclosed area filled deeper and deeper with mud, causing the surface water to overflow the dyke. After about half an hour, I had raised the level of mud inside the enclosed area enough that all of the surface water had been replaced with the mud. Viola! A nice oval area of mud sitting in a large pond.
I had to be careful not to disturb the edge of the area, but I was able to get into the middle of the mud oval and it was amazing mud. Of course rinsing off was easy, because I was already in a large pond, so I just had to clean up and get back in my kayak.
So if anyone discovers a similar situation where mud is under some water, this is a technique where you can create a mud pit which is level with the surface of the pond.
In the photo, the duckweed sits over about eight to ten inches of water. The mud area I raised from under the surface was about four feet deep and very soft mud, like chocolate pudding only warm on top, cold underneath.