You don't need to worry about anything. The water pump is a static unit, meaning it does not move...it is firmly attached to the engine block and it does not need to be removed to install a new belt. This is assuming of course you're having the belt replaced because it's bad, not because it snapped due to the water pump seizing.
If it's just the belt it's a matter of using the "belt tensioner" pulley to release/add tension to the new belt. It takes 10 minutes max to do this, if they're skilled at all it should be closer to 2-5 minutes...gives you an idea of how simple it is and how low cost the replacement should be.
If your water pump is seized (pulley on the pump won't rotate) and the belt broke because of it you're entering a new ball game all together. At minimum the pump will have to be replaced, if the car overheated you need to have a compression test run to ensure the engine head did not warp.
Hope this helps!