I went fishing with my grandpa and my grandma's cousin today. We tied the boat to the riverbank and saw a water moccasin swimming near the bank in the shallow water. My grandma's cousin started flipping out how we needed to move away from the "poisonous snake."

I know that snakes are venomous, not poisonous, but right then I realized I had no idea how to explain they were one over the other. So while I feel stupid for asking this question, because I have a feeling it's going to be a simple answer that I should know, can someone explain this to me, please?