as long as your dress is a dressy dress, short is just fine. If you have a long dress, by all means bring it if you want to!
As for the falls & Sting Rays, wear your bathing suit with water shorts over them, water sandals, water shoes or canvas tennis sneakers to climb the falls.
If you wear glasses, make sure you have croakies to go around your neck while climbing the falls,you don't want to loose your glassess.
Leave your towels and belongings on the boat when you go into the water with the Rays.
You might want to rent a locker at Dunns River Falls or leave your stuff on the bus if you take a tour.