A water stone is friable, meaning that new surface is constantly being exposed. A Japanese waterstone will work faster than a sandpaper and last much longer. The flatness can be an issue but if you lay the sandpaper on a tile or a piece of glass you're going to be at least as flat as the water stone. And of course, the water stone isn't gonna stay flat; you'll need to flatten it periodically, either on a stone fixer, a bit of sandpaper of another stone/plate. A DMT or Atoma works great. I use a DMT XXC for flattening.

Yeah, the Atoma is better but due to the lower price I still have eight different DMT plates and I still use them, but mostly for flattening my water stones. I don't really dig diamonds all that much for actual knife sharpening except at the really low end. They really excel at cutting the initial bevel. Above the 140 Atoma I generally switch to stones.