Copper sulfate is used extensively in treated lumber and will dissolve out fairly readily while submerged. Copper is not a friend of plant life, but I don't know how much will leach into your water. Water will preserve the wood where it leaches out, but the junction at the air/water will rot quite quickly with today's treatments. Older treatments were nasty stuff but would last much longer in pond conditions I am told and didn't leach out as much.

For my dock I purchased a special kind of treated lumber where they seal in the preservatives and add something else that stabilizes the copper so it is not soluble for use in fresh water such as waterways. 84 Lumber can special order it, and it is only a small adder to the wood cost (10%). I will see if I can find what it is called again and re-post.