While working to replace the timing belt I noticed the small rust/water mark on the pump's shaft and on the crank pulley (See pics). Is it starting to leak?? Shall I replace this water pump ?

 

I always work on repalcing the water pump every second time I replace my timing belt. This has been the case in all of my cars, bearing in mind if the water pump fails, you need to put another timing belt on it.

 

If the water pump is not leaking I would not change it. You can usually see coolant somewhere in there if it is. It may not be a bad idea to change it if it's near 15 years old. That dab of rust may have been there from when it was installed.

 

I'll second changing it every other time I change the timing belt. I don't see any leaks on your pictures, but if you are uncomfortable, I'd change it to be sure.

 

For the price of a water pump you won't have to go back in there later and take it all apart if it is leaking or if it starts to leak anytime in the near future. It's cheap insurance to do the pump and belt together.