Couple of issues...
For those noting the weight of water: 1 kg of water weighs exactly as much as 1 kg of Halon 1311. (I'm not being snarky; I'm pointing out that what matters is "how much fire extinguishing power does 1 kg of water have relative to 1 kg of halon 1311?") I don't know, and a 30 second scan of a couple of articles didn't enlighten me.
For those noting the concern of water turning to steam in a closed cabin and creating overpressure: The same is true of halon 1311 - it comes out of the cylinder as a liquid and rapidly expands to a much higher volume of gas, same as water does when it encounters heat.