The theory I heard about the island was that they were going to put a few VERY exclusive honeymoon villas there. Read very expensive, very private, very luxurious.
I don't think this would be a danger at all. The waters around the island are fairly shallow to a point -- while I'd be slightly concerned about alligators and such, if that big a fire erupted on the island and Disney couldn't evacuate me off the island quick enough, I'd just walk out into the water a ways. But being that these villas would be fairly exclusive, I'm sure they'd have a set off boats that would hold everyone to get them quickly and safely where they needed to be. I think you'd be in more danger of a fire if you were staying in the Contemporary towers upper floors than on the island. I'm also sure reservations for the island villas would come with the caveat that due to inclement weather (hurricanes, etc.), Disney may be forced to move your accomodations to one of their resorts.