It would not be wrong to say 'six bottles of 1,5 liter of water' as that is what the product is. People often do not read what is literally on a label, for example a bottle might say 1l, but people will say one liter.
An alternate way to read the label is "Water 6 by 1.5 liter" or "6 by 1.5 liter of Water"
"X" represents the multiplication symbol, sometimes spoken as "times". 'In some contexts, it isn't spoken at all, and we would just say what it means using common language. The same is true of most abbreviations - we read what it means rather than what it says.
"6 x 1.5 litre" means that, inside the package, there are 6 bottles, each with 1.5litre of water ...