立一个牌子就没事了。

Private Pond, No Swimming, No Diving. 

As a land owner, you have duties towards trespassers, guests (to homes), and invitees (customer to a business premise).  

Your duty to the trespasser is warn of all hidden hazards such as high voltage, traps and water that is not easily seen.  Once you publish a warning, your duty is satisfied. 

Your duty to guests and invitees are higher, that falls under negligence law that you can google yourself.