Keep an eye out for the appearance of extra costs on your bill. It is worth checking the menu to see whether it will be included or added. It is not compulsory to include VAT in prices quoted but you should be aware of the possible hidden cost or the bargain meal you thought you had found might not turn out to be such good value after all. It's best to drink bottled water as a rule. Water from the tap is not harmful but doesn't taste that good.
Carbonated water is called 'agua con gas' and still water is 'agua sin gas'.