For a dock, I wouldn't worry a bit about using treated lumber. For the type of project you are talking, I would stay away from them in a pond where I wanted to eat fish. In my opinion, creosote and copper napthenate aren't overly concerning from a health standpoint, but there is a 3rd chemical that is commonly used in treatment of industrial wood products called Penta (I don't know if that's the actual name). Apparently, that stuff is extremely toxic and has been banned from use in many countries but is still fairly common in the US. In a fairly low dose, I probably still wouldn't worry about it due to dilution, but a project like you're describing could potentially introduce a huge amount of that into your pond and likely be hazardous to your fish or yourself.
