Some bigger boats have landing pads for helicopters...
They probably are first deploying all assets to rooftop rescues. The Superdome may be gross and uncomfortable, but it is better than a roof, and people can temporarily survive there as a staged shelter, no one intends for it to be permanent. There was a report late last night that the sick in the SD were going to be moved to the convention center, which is better suited for that. So there are some things going on that aren't getting reported much, it isn't all unplanned chaos.