Watched my neighbor try to save money with untreated posts along his creek bank. They looked fine going in that first fall. By the next summer, you could push half of them over with your hand. The bottom foot was just spongy, rotten mush. He ended up having to redo the whole line with pressure-treated wood. Honestly, it was more work and cost him way more in the long run.
My cousin did the same thing with a split rail fence near his pond. Thought he was being smart skipping the treated lumber. Two springs later, a good wind knocked over three whole sections like dominoes. The posts snapped right at ground level, totally eaten up. He spent that whole weekend digging out rotten stumps and swearing at himself.