Re: would moth balls keep a cat out of my boat?
Why don't you tell the neighbors to keep their cat of you property. It is a reasonable expectation for dogs, why not cats? If they don't like your suggestion, suggest to them that the cat trap (coon trap/skunk traps work great) in your boat will do a fine job of transporting fluffy to the local animal control facility.
I love my neighbors, but my Sister in law and their daughter is very allergic to cat hair. They get an almost immediate skin reaction if it touches them. We had a neighbor cat that seemed to love to sleep on our back porch swing (fenced back yard). It was unfair to our dog to keep him from chasing the cat, and unfair to my family to have to deal with the cat hair. I had a friendly conversation with my neighbors, which was laughed off. When the conversation became my serious, they got my point. Since then no problems. I never did have to resort to a cat trap, but I would have done it if the neighbors did not take care of THEIR problem.