Lassig, Here's how I did mine and they have performed well. I used steel posts and welded a steel pin to the bottom of a steel plate and welded my post on top of that. The pin/plate base could easily be adapted to a 6x6 post. The pin easily sunk into the clay and was added for extra stability to keep the bottom post from trying to slide down the slope of the pond bottom during installation. The plate should be sized for the weight the post will be supporting and the soil type it will be resting on. Also this design does not add any significant lateral stability to the structure it is supporting like a sunken post would.
Beside being installed after the pond was full, I designed this base this way because I wanted my dock very close to the water to I could climb out while swimming without a ladder or easily get on/off of a raft. My fear was that during winter ice would heave the dock up and soil would fill under the bottom of a sunken post and cause the dock to get out of level. This design allows to the dock to heave up and then settle back down where it belongs.
I'm probably way overkill on material thickness/weight but there isn't much scrap light stuff in the bridge business.
