If you are CERTAIN that the falls is the only place you would use the Aqua socks, then renting at the falls is probably the way to go. However, before you do, consider that they are great for other things as well- walking on the beach/in the water. Going on the sting ray encounter, just going to and from the ship to the excursions. (Wear the socks to walk in and you don't have to worry about shoes... if the socks get wet.. no problem.)
Buy a cheap pair at Wal-Mart, they probably will cost you the same as renting them and then you can use them through the whole cruise.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007