You do not need to fill your SodaStream bottle to the line, nor does the nozzle need to be in the water to carbonate it. The line is there to serve as a maximum fill line — exceed it and the liquid will overflow in carbonation, creating a serious mess.
Suffice to say, the height of the line is relative to the size of the bottle you’re using — the maximum fill is higher on a larger bottle and vice versa. All official SodaStream bottles will have one, usually on the opposite side of the logo.
It’s unlikely you’ll do any damage by exceeding the fill line or by not meeting it. The amount of carbon dioxide used will remain the same though, so we recommend meeting the fill line each time to squeeze the most out of each canister.