This doesn't answer the original post asking about contructing a dock in a exhisting pond. I put two pilings (2 1/2" pipe) in when I constructed the pond We had a track hoe on site so used it to push them in. They go down to bedrock. Maybe the best thing I used was engineered steel trusses to span from the pilings to the shore (24" clear span Made it alot easier to not have to put in as many pilings. It is 12'square then drops to 8' x 12'. This is strictly for looks.

Some of the locals on a lake pound in steel pipe from a flat bottom boat. I am not an engineer, but the load bearing seems a litle light if you can't get to bed rock or have a large "foot". Maybe a 2' + concrete disk.

Mine is not complete, but steel is and wood frame is started. Only thing holding progress is $ and decision on whether to use engineered plastic or wood for the deck. Would be glad to email a picture of anything you want to see in detail.

Good luck on your project.