Well...it sounds like the last person in your situation stopped paying the property taxes and the county sold it at a tax sale. The only problem is that someone needs to buy it at the sale. You should look into getting liability insurance in case there is an accident around the water. Good luck.
You are stuck owning this pond unless you can get someone to buy it from you. Otherwise, if you stop paying the property taxes on it for 5+ years, the county could possible foreclose on its tax lien.