On the point of the legalities, keep records of what your doing to keep the peace. If it ends up in small claims or civil court, being able to show you went above and beyond will defray any claims he has. As far as the drainage is concerned, you cannot claim adding the pool and other items has not changed your properties drainage - as a result, he can easily claim you caused problems - it may be incorrect but the change makes it a real possibilty in the courts eyes. If you have to add drainage pipe and a flow out grate or pop up flow out cover do it. Again, in court - being able to say do did everything possible to address his complaints and went above and beyond leaves him claimless.

In my case, things ultimately eroded to the point where the police and the Township Building Department were called several times. Once they saw what I was doing to mitigate the situations, both told me they were surprised how much I had done, apologized to me for the trouble and went next door to tell her she didn't have a leg to stand on and that she should be grateful she had such a good neighbor - I know this because I sat and talked with one of the officers over a coffee on a following visit. If I had to supeona the Officers or Township in a lawsuit, she would have looked awfully silly. I eavesdropped on conversation she was having with a lawyer in her backyard. He told her she could not win with the amount of effort I had put into mitigating her complaints.

Again - stay above the pettiness. Go above and beyond and do not respond by being even slightly negative to their pettiness.