NEVER NEVER NEVER use pressure treated lumber. The treatment in the lumber is copper, chromium and arsenic (CCA). This very easliy leeches out of the wood as well. Stuff that fish dont like will end up int your water and most likely kill them.
A very good wood to use that is extremely resistant is ipe. It's a south american wood that is more dense that water and therefor sinks, but at the same time, since it is so dense, it will not rot. The harder and heavier the wood, the better.
Jim