It is always best if you can address this in a friendly discussion between neighbors. If that is not possible, the first thing you should do is some research through DES records to determine when the dock was built and if it was legally constructed in it's current location. If the dock was legal when it went in then DES will not apply the 20 ft. setback to it retroactively. If your research finds that the dock was not legally constructed then perhaps try approaching your neighbor again about reaching a mutually beneficial arrangement. If that doesn't work then you should file a formal complaint with DES using the form found
here.