I put in 6x6 inch green treated posts 15 years ago for my pond deck/gazebo and I can't see any degrading to the post in the water. My holes were about 4 1/2 feet deep. I flared out the bottoms for a bigger foot print foundation and dropped in a half bag of dry concrete. I pounded clay around the posts as I filled the hole using a sledge hammer. I set post the same way for my pole barn I built back in the 70,s. I remember reading not to pore concrete around the post because it would make them rot, but I don't remember why.
