It may not have actually rained for the great majority of scenes where things are wet. It is very common in movies and tv shows to have the crew wet the streets before filming. I can't say that I necessarily agree, but the belief has always been that wet pavement look better on camera, day or night. Each time you see wet streets during a Five-O episode, look at the rest of the surroundings. You'll often see everything but the street (and maybe exposed sidewalks) is dry!