Although I have practiced law in Texas for nearly 40 years, I would caution you that the law of the state where an event occurs governs the issue. I do not know Ohio law; only a lawyer licensed to practice there could give you authoritative legal advice. The pond could be viewed as an "attractive nuisance," particularly if it were located in an area easily accessible to children and was located on a lot located in an area known to have children in close proximity. Even a fence might not be sufficient if it were not formidable enough to keep a child out. Insurance is advisable, but there are no guarantees.
