Hi there - we went to Dunns River Falls in the summer 2005. I believe that they were renting water shoes to people for around $7 back then. However, we brought water shoes from home and we were kind of glad to have our own. I got them from swimoutlet.com - go to this site and search for water shoes. They come in all different prices, but I think they start as low as $9-10 dollars. Make sure they have a good grip on the bottom - it can be very slippery at the falls! I also saw people there wearing old sneakers that they didn't mind getting wet - problem with old sneakers is that the water doesn't drain out of them (very heavy).
The water shoes are also great when you're at the beach and the sand is burning hot, or when you want to go in the ocean and you have to walk on broken shells or stones (not to mention stepping on crabs).
The guides at Dunns River Falls will offer to hold your camera for you in their waterproof bags, which is a good idea (unless you have your own waterproof bag that you can carry easily - you need your hands and arms to keep your balance). They will also pull out your camera and take your picture along the way. Just be ready to tip them at the top of the falls - bring some small bills with you.
Have a great time!