In reply to obi-wan nick b:
Might that not just be grit 'n' grime from the puddle? The tolerances in disc brakes are close enough to cause even pretty small grains of gunk on the rotors to rub against the pads. Aside from avoiding puddles altogether, maybe ride through a cleaner one, or wait for the dirt to wear itself off. Or give a squirt or two from your water bottle, maybe