When cutting granite does a diamond blade need to be wet?
Whether you use water or not is entirely dependent on the blade, not the material (unless you are cutting something soluble in water).  Some binders used in diamond blades and bits take heat better than others, and the trade off is how the cut works (aggressive material removal, smooth finish, ability to cut heterogeneous materials, etc.)

Do not use water with a dry cut blade, and don't run a wet blade dry - follow the recommendations from the manufacturer. AKA, RTFM.

If you do it incorrectly, your blade will either not cut, wear out too fast, burn the material or the blade sintering, or just blow apart entirely.

And remember, lower speed and less pressure make for a slower, but safer, more precise cut with far less dust and wear on the tool and the blade.