i have played around with water and granite because i built a milling machine from granite, and one problem i ran into was skin oils on the granite surface preventing epoxy from adhering to it. when the granite is wet, you can heat the surface up with a torch and the granite will condense water from the propane torch quickly, but it doesn't take much heat to dry it off completely, which again points towards very thin water absorption layer.
water and oil penetrate into the granite but not very far. you can wipe the surface off, then sand it with 240 grit diamond lapping plate by hand and within a few seconds you have dry granite. but obviously it varies a lot from one type of granite to another.
orbital77's claim that he thinks it does.. is most likely going to be due to the thermal effect of water cooling off the surface. i have read some papers on the creep rates of granite and other papers as well claiming as much as 1% water absorption by weight.. but they were small samples and we're talking soaked in water for a long time, or soaked in water with pressure to force water in.. vs drying out the samples.
I have read posts on this forum about 1-2F temp difference from one side of surface plate vs the other (due to ambient light or other heat sources) being a significant source of error when measuring plates for global flatness.
grinding a granite parallel on a surface grinder will easily cause far greater temp differences.
heating up a thin granite parallel to bake all the water out.. then soak it in water for a day and wipe it dry, my guess is you could easily measure .5% water absorption. if its on only one side of the granite then yeah it might move just a little bit. but if its a thin granite parallel flushed with water in a grinder, its not going to matter since you're going to flip it over repeatedly to grind both sides flat.
this site claims .01 to .8% water absorption.
Quality Control 101: What Is Absorption? | Kafka Granite, LLC