I know that marine grade posts exist, but I have no idea where to get them.
15 years ago, when I was building my house, the grade of PT sold by Home Depot was "not suitable for ground contact". If I went to a regular lumberyard they had a grade which was suitable for ground contact, so there used to be a difference.
The chemicals used to pressure treat wood have changed recently, so I don't know if this is still the case.
Go to the big box store of your choice and carefully read the stamp (not the stapled-on tag) on a piece of PT. Write it down, because there is a lot of verbiage in that stamp. Youmay have to look at a few pieces to get it all, because not all of the words come out clearly on every stamp.
Then go to a lumberyard and read a few stamps. This will tell you if there is a difference.
I have never met any clerk at a big box store who knew the truth about the PT they sold, and very few in a lumberyard know it either, but the stamp tells the story.