The situation with the water shortage in and around Cape Town is indeed concerning.
I e-mailed our TA about the shortage (we are going to KwaZulu Natal, Sabi Sands, JoBurg and Botawana in June) and I was assured that the shortage is just around Cape Town. However I have read that the shortage is spreading further, towards JoBurg and Pretoria. So I am not sure if / how we will be affected.
My hope is that that the winter rains will start before the June ABD departures and the trips won't be affected. I am sure that ABD must be re-thinking the Cape Town portion of the 2018 itineraries. From what I have read I personally wouldn't want to visit with the restrictions in place (2 minute showers, no washing hands in public restrooms {bottles of sanitizer are on the counters}, no flushing, etc. -- I just wouldn't want to do it). But I understand that for most the current water restrictions aren't a big deal at all.
I will say that if any company can pull off an amazing trip within the current restrictions, it's ABD! And having been to Cape Town, I can say that it is extremely beautiful and well worth visiting.
I just returned from 10 days in JoBurg and there were no water restrictions, despite what I had read prior to traveling there. Don't know if that helps in your planning, but wanted to put it out there.