When the water pump goes, it'll leak out onto the timing belt, weakening it whether its brand new or not. Even newish belts can snap causing a lot of expensive engine damage. Because the water pump is behind the timing belt, it'll be cheaper to replace it now rather then pulling the belt apart again later down the track. While you're there, you may as well replace the timing belt, tensioners, crank and cam seals, and possibly rocker cover gaskets as well considering they're off anyway. Spark plugs wouldnt hurt either. Do it all together to save from pulling it all apart multiple times. Up to you though mate. I did it all myself, so labour wasnt an issue.