Tim W,
Since no one with experience has answered, here is an answer from someone with limited experience with Walnuts.
In Missouri, where I grew up, we would never have a garden anywhere near a walnut tree...because of the reasons you mentioned. It stands to reason to me, that the same logic would apply to ponds.
Having said that, I wish I could grow them in Texas...I love the trees....but not around a pond.
Further, you do not want any trees in your pond's dam....none. They cause structural damage over time and leaks and big problems.
