Woody - Have stuck them in soft clay before where it took sticks, PVC, hard ruddering, full throttle, and 2 men pushing/pulling on the bow with the ruddering to get it to move up onto the sticks.
BEWARE of dredge mud the Corps of Engineers pumps from the navigation channels. Until it sets up and dries, it is very bad. Fun to cross when fresh, but as the water settles out, it thickens, and if you stop you might be surprised. The weight of the boat causes you to sink in some and it forms a suction like glue. It may have been the boat we were in but, typically that boat would run dry real easily. The only other time I heard of him sticking it was in some willows. (Another warning, if the saplings don't bend over as you go through then, them may stop you real suddenly).
An old timer told me the mud on river deltas after spring floods came act the same.
Try Ruddering first, then the sticks and PVC, unless you have some lube. Lots of boats in Louisiana have sprayer systems built in. An ag spray 6 to 15 gallon, with nozzles under the bow that spray downward will help alot. As you hit hard running stuff, push the spray button, and you get a little extra. A hand held 1 to 2 gallon garden sprayer will work just as well. Wisk or Dove detergent about 8 oz per tank works fairly well. Never tried the silicone stuff, but should work wonders. Diesel is very slick and you get some very bad fines with its use like Big Daddy said. It will get everyone in the state up in arms about airboats and the next thing you know is they will all be lobbying to ban airboat because of the envirnomental issues. So DON'T USE IT.
Also, don't use the stuff to allow you to get somewhere, cause you might end up looking for a buddy with a swamp buggy to pull you out. I have seen that many times in the seismic field, some boats would run dry all day and others would try to keep up by using a little slickum. I remember at least two motors that went to pieces on boat that were trying all day for several days to keep up with others by using sprays. Just learn your boat and stay to those areas. Good Luck