Your method of asking was very confrontational, even if you did start it politely. You challenged them to "back your claim" which wasn't their claim. You could

Your method of asking was very confrontational, even if you did start it politely. You challenged them to "back your claim" which wasn't their claim. You could have asked something like "do you agree with the Japanese claim that the water is safe to drink? Would you be willing to drink it?"