I don't generally like to skate in the rain, as it almost always results in rusted bearings unless I am very diligent about drying them off afterwards.
But water is clever and has plenty of other ways to get onto the roadway. Think of: snow melt, dripping air conditioners on cars and trucks, overzealous lawn sprinklers, leaking fire hydrants.
And sometimes a little water changes everything. Good examples of this are the road surface markings that denote traffic lanes, limit lines, pedestrian crossings and the like.
Many of these are not actually paint. Instead, they are composed of a substance called thermoplastic, which, as the name implies, is applied hot to the road surface. Once in place, they have pretty much the same gripping properties as the asphalt or concrete roadway they are applied to. Except when they get wet. In that case, they lose pretty much all friction and may as well be sheets of ice.
Fallen tree leaves on the roadway present similar dangers. When they are dry, they are fine to skate over. But when they get wet, they are like miniature patches of ice or oil that provide no traction at all.
Just keep in mind that water can dramatically alter the gripping ability of some materials.
Tim Monroe (a.k.a. Tristo) is an avid inline skater who has managed to avoid major injury despite decades of urban skating in cities like San Francisco, Oakland and Boston. In May 2013, he finished his skate of all 850 miles of Boston streets. In 2006, he completed his skate of all 831 miles of Oakland streets, earning a commendation from the City Council, which praised him for his "unique and extraordinary physical accomplishment." His exploits have been heralded in children's books and documentary films. When he's not skating, he might be working as a software engineer for Apple. Tim currently splits his time between Cambridge, MA and Oakland, CA.