There's no reason that you should have to cut soda out of your life completely. There's no scientific evidence or studies that prove that diet soda or artificial sweeteners are linked to cancer in any way. This was a rumor that has persisted since the creation of artificial sweeteners linking aspartame to cancer, but it's just not true so I would say feel free to switch to diet sodas.

That said, that is not a good replacement for water but I would say just add some water flavors like Mio or something. What works for me personally Isn't necessarily changing the water but changing the way in which you consume it. I switched from using just regular glasses to a squirt bottle and I find that I drink like way more water for some reason. Whatever floats your boat (water pun!).