Generally speaking, water snakes or black snakes are not poisonous, but what you said is a gray snake, probably a viper. They are small poisonous snakes that are widely distributed in China and have the same toxicity as cobra, and widely live in rural fields.

Chinese name: the Yangtze River short-tailed Agkistrodon halys, the common name is Qicunzi, and the scientific name is Agkistrodon halys, which is a little less toxic than the ring tail.