In respects to granite used in home countertops and similar applications, it is highly unlikely that water will damage granite countertops.
More Info: Granite is one of the hardest and least porous natural stones making it a reliable, durable surface for a kitchen countertop. It is resistant to stains, heat, and scratches and can withstand much abuse.
That being said, it is still a natural stone and though tiny, it is porous meaning that liquids can ultimately permeate the surface. The good news is that under normal home conditions with stable temperatures, any water that is able to permeate the surface will likely leave a dark stain, but will ultimately dry.
It is worth mentioning that you do need to be careful with other liquids that do not dry, such as oils. When these liquids manage to penetrate a solid granite surface, they are much more difficult to remove, if at all.
Granite in Nature Is a Different Story
When considering granite in nature, water has the power to damage almost any rock through both physical and frost weathering.
Physical weathering is the process by which forces of nature break down rocks and soil. Water from rain, rivers, and the ocean all have the ability to break down granite over time.
Frost weathering occurs when water penetrates the cracks and pores of granite, which when frozen, can expand and cause fissures in the rock. Over time, as this process continues, water will ultimately break the rock apart.
Granite Countertop Care.” Granite Countertops, Granite Tools, Granite Slabs, Granite Information. N.p., n.d. Web. 19 Oct. 2010. http://www.georgiagraniteassociation.com/html/granite_care.html.
” Wageningen UR – Wageningen University – Land Dynamics .” Wageningen UR – Wageningen University – Land Dynamics . N.p., n.d. Web. 19 Oct. 2010. http://www.lad.wur.nl/UK/Research/LAPSUS/Modules/Frost/.
“Granite Countertop Stains.” Ask the Builder – The Home Improvement Resource. N.p., n.d. Web. 19 Oct. 2010. http://www.askthebuilder.com/N8_Granite_Countertop_Stains.shtml.