the mass of a bottle of mineral is
I am pretty sure that 700mL of water will be 700 grams of water. The density of water is 1, so you use the equation density=mass/volume. You would plug everything in but mass (which you don't have)....
If the bottles are identical, then the bottle of water has more mass in it than the bottle of steam.
That depends on the bottle. Water in the US is sold in containers ranging from .5 liters to 5 gallons in my local grocery store.
The mass of the bottle will remain the same whether it's full or empty however, the weight will change.