I just finished my dock out of 4x4 treated posts. I would have used 6x6s, but I was builing it by myself. I buried each one approx 2 feet deep with at least one big bag of quickcrete on each one. It's T shaped with the main walk 5' by 30' and the T on the end 5' by 15'. I plan on using treated 2x6s for the decking when the water level stabalizes. Anybody know if it would be of any benefit to treat the posts with an extra layer of sealant before the pond fills up?