The Shimano BB is indeed sealed and water should not be able to enter the cartridge itself. However, the way the BB mounts into the BB of the frame is not *necessarily* water tight, provided the BB is not greased liberally when it's installed.

The clicking I experienced originated from where the left side BB nut snugged down around the BB body. It's a simple interface between the nut and a raised band around the cartridge--if this intersection is not greased fully, the two bare metal surfaces can slip/stick w/ every pedal stroke. Water will exacerbate this condition, and water CAN enter this intersection if it's not fully greased--it's the way the Shimano BB is designed.

FWIW (and to show what I do/don't know ) I've never heard of H2O running down the seat post either...