Riding in the rain on a bicycle is very much like driving in a car in the snow. You need to leave more distance to stop due to wet brakes, and you won't be able to turn as sharply. Keep an eye out for little rainbow-edged patches of oil on the pavement - these are extremely slick and will crash you in a split second if you try to lean over on one. Don't worry about hydroplaning, though. Bike tires are narrow enough that they'll cut right through puddles and hit pavement.