The liability of a moored dinghy colliding with another moored or anchored vessel brings many complicated admiralty laws into question. If you ever found your way to admiralty court each on of those vessels is its own hull with rights and burdens. If there ever was any actual damage. Most of us would get dinged for failure to maintain an anchor watch. Assuming an adequate anchor watch the boat dragging anchor is always at fault. Two boats swinging together share the liability. I do not think the no boats over X would play when two boats swing together securely anchored. In the case of the outboard the 12 foot anchored vessel would be the party anyway and your anchor watch should have taken efforts to keep your vessel clear. Same with the dinghy. Each covers their own shared responsibility damage. The jerk may have been legally correct.