It isn't that most grease is water friendly and only some are "waterproof". Pretty much all greases are not going to dissolve away with water but that, as Ironfish653 said, the water and grease mix in an emulsion. At best the added water and thus added volume of the fluids will see some bleed out and reduce the actual grease amount in the bearing area. The water off the road isn't just water but contains chemicals not friendly to your bike and carries grit/grime, think of a polishing tumbler and its slurry.
Phil Wood long ago talked about how water and grease mix and how that leads to bearing failure. Back then it was known, with a wink and smile, that their claim of "waterproof grease" was a total marketing hook. It was their industrial bearing's better seals (compared to the metal dust caps common then) and proper preloading that made them last longer (and thus the dawn of "sealed bearing" began
) Andy