Note that there are different treated lumber grades. There are grades suitable for ground contact, and water submersion, meaning they use a form of copper that does not dissipate into water like the cheaper Home Depot varieties do. 84 Lumber will supply the various types you need.
My dock pilings (4x4) of water submersion grade lumber is growing some bryozoans. If anything is a testament to being safe, it is those.
I would stay away from Creosote. Just because I hate the smell of that shiz when it gets hot in the sun.
