If it's something really serious, like a hurricane, Marriott issues a statement and hotels impacted by it waive cancellation fees for confirmed reservations. Airlines typically also waive change fees and cancellations fees under such circumstances.
If it's just bad weather — thunderstorms and winds are common in Florida this time of year — then I would expect regular cancellation fees to apply.
So the question is, will Marco Island just experience bad weather around Labor Day? Or will it be a serious situation, possibly involving evacuation of the island?
It makes sense to me that a reservationist at a call center would recommend that you call the hotel directly if there's not an official waiver in place.
Let's face it. One of the reasons for a 7-day cancellation rule instead of a 2-day cancellation rule is to avoid having half the guests cancel if the 5-day weather outlook calls for rain.