Pure water by itself is fundamentally halal. If there are haram impurities involved, the water would also become haram; not because of the water itself, but because of what's in it. So, for example, if it was drawn from a small pool in which someone was storing their dead pig carcasses, we shouldn't drink it (most would chalk that up to common sense though).
It's always nice to know that there were no pig carcasses involved in the production of your water, but I would call the certification overkill in this case.
Of note, if the water is not actually pure bottled water, but rather some brand of flavoured/enriched water, there is an increased chance that the additives themselves are haram. Given that a number of common additives are of animal origin, the certification makes more sense in such cases.