Bottled water is fine but you should check the sodium levels and get one with low sodium - there will probably be one advertised as being suitable for babies (although, tbh, as you are only going on holiday, any bottled water will do as none are actually salty!)
The tap water in Spain is safe to drink although in some areas tastes quite strongly of chlorine. Where DM lives, the water has a high concentration of heavy metals (caused by mining in Roman times) and so it not recommended for very young babies or children. DM buys a brand of bottled water called Bezoya which is supposed to be very pure. Personally, I always drink the tap water; DM has a filter to take the chlorine out otherwise your cup of tea tastes funny. Boiling the water won't take out the minerals or the chlorine.