Do you have to drain all the sugar water from water kefir regularly?
Technically, you can. 

The advantage of cleaning is that you can start with the right amounts of kefir, water, sugar and whatever you add. If you are aiming for a certain taste, starting from scratch makes it easier to match again.

If you don't, there won't be a problem (at first), but as time goes by the chances of contaminations in the residual water will grow. Those contaminations can affect the grains.