john's right, the labor to do the water pump is a major PITA by itself, but it's a breeze when done while you're in there already for the timing belt. most water pumps (speaking generally here) will last one and a half times your timing belt. in other words, if your t-belt will die at 100k, your water pump will die at 150k. makes sense to do it as preventive maintenance during the t-belt then wait for a failure down the road.