it's been tried before and was not economically feasible at the time. an iceberg has a mass in the millions of tons. for comparison the largest aircraft carriers displace about 100,000 tons. so an iceberg would require 10 of the world's largest tugboats for every million tons, maybe more since the underwater shape of an iceberg is not very hydrodynamic.
it could probably work for relatively short distances. i keep forgetting SA is close to antarctica. if you haul one big iceberg of 1,000.000 and compare the amount of ice that arrives at the destination, to hauling 10 icebergs that are 100,000 tons the same distance in the same weather, the million ton berg will deliver more water than the 10 smaller bergs. the reason is the smaller ones present a much larger surface area to the ocean and the air. i wonder if you could just implant a bunch of diesel engines on the berg. making the engine mounts out of pycrete might be a viable way to transport icebergs.