I agree with your interpretation because of the serial commas. You may be able to deal with your manager's objection by placing a colon after "when".
I often make up fliers for our safety department. Recently I completed and posted an advertisement regarding hydration. Upon entering work the next morning I was approached by the Manager and asked to take them down. The reason he gave was due to one sentence, which can be misconstrued to state something that was not intended. I would like to get clarification of the proper usage of the word "or" in the sentence. Below is the sentence used:
Drink extra water when you exercise, spend time in the sun, or drink coffee or alcohol.
I see it as "Drink extra water when you exercise. Drink extra water when you spend time in the sun. Drink extra water when you drink coffee or alcohol."
He saw it as "Drink extra water when you exercise, spend time in the sun, or you could drink coffee or alcohol as a substitute for the water".
I see how he could see it the way he did, but I tried to explain that there was not a (.) used within the sentence, therefore it would be a continuation of reasons for drinking extra water.
I am going to change the flier but would like to get an answer, as to who is correct in their interpretation.
Thank you for any information provided.
I read it similarly to the way your manager did. It's ambiguous at best. As it stands, it lists three alternative actions you can perform. "You can/should i) drink extra water when you exercise, ii) spend time in the sun, or iii) drink coffee or alcohol."
I agree that listing the clauses after a colon would be appropriate - not simply to please your manager, but to make it unambiguous to the intended viewer.