The advantage to the earth augers is you just keep adding extensions on and screwing them into the ground until you hit cohesive enough soil to resist the specified torque. I had priced them a couple years ago to do a similiar task, cross a peat bog to get to my lake. The labor quoted was $300 per piling, then add on the piling cost. The contractor I found uses a driver that is hydraulic driven and fits on a skidsteer, but the website states handheld drivers are available. I could not find a rental place for the driver, and couldn't find anybody to sell me the augers without using an authorized contractor - too much lawyer exposure I guess.
For just a simple dock walkway, you could probably use the cast aluminum screw heads that I've seen in the local ACO hardware store. They are made to go into 2" aluminum pipe, but you could probably use 2" schedule 80 galvanized steel pipe. The advantage is that the schedule 80 could be threaded, while the ACO store aluminum pipe is not theaded. Extensions could be added on until solid soil is reached. A local marine/dock store sells a large rachet-like device to turn these into the ground, but this is by hand, and looks like a lot of manual labor. Somebody that is more mechanically inclined could probably convert one of these into a hydraulically driven device.
Another alternative is to use a large steel plate, ~ 24"x24"x1/2", as a mud pad, and make your cross pieces supporting the walkway adjustable so you could raise the walkway as the mud plate slowly sinks into the ground. This method would take a lot of diagonal cross bracing to stabilize the walkway.
