The seat post Zed can be hidden or at least made more presentable with black marker pen, but shimmed posts I always put the slit in the shim opposite to the slit in the seat tube, the idea being more of a labyrinth for water to travel down and the fact that most of the nasty debris came from the back wheel, it was one of the reasons I liked Saracen so much they thought about that aspect and put the slit at the front of the tube on '92 onwards models. But of bottom bracket woes and water, I doubt water will affect a cartridge BB in terms of operation and perhaps not older styles, but the shell could rust, but if you have the under BB cables and anylon type thing for the cables to run in, just undo the screw to let any accumulated water run out. But as it is when I do a BB replacement, before fitting the BB the inside of the shell is well greased to stop water eating into the metal. The BB being of course the lowest point where any water should accumulate. Also in maintenance all the gas holes in the frame get a liberal squirt of lubricant, as it occurs gas holes are where water will ingress.