Reading through several articles (for an example, and another) as well as the Ministry of Tourism site, Cape Town still wants (needs) tourism to continue. They depend on the revenues to help with the crisis. However, there will be noticeable changes you will see: you'll be asked to hang your towels, for instance, instead of having them washed and replaced daily; you will see reminders and hear on the radio shorts designed to bring awareness to the crisis for things like in and out of showers in under 90 seconds, etc.
One thing to note, is that the actual crisis, for now, is limited to Cape Town itself. Travelling inland an hour gets you out of the crisis zone.
So, if you are to believe what is being written, they still want you to head their way for your vacation, but you should definitely expect to do your part to conserve, alongside the residents.
Also, the peak of the tourist season is November to January, and even during that period it is estimated that tourists only account for just under 1% of the total water usage. As you are planning on going in April, the overall impact would be even lower, but it is unlikely that there should be any concern by you then.