Actually, that's wrong by three orders of magnitude. One cubic km has 10^9 cubic meters. Each cubic m has 1000 kg of water, so one cubic km has 10^12 kg. If you

Actually, that's wrong by three orders of magnitude. One cubic km has 10^9 cubic meters. Each cubic m has 1000 kg of water, so one cubic km has 10^12 kg. If you now have 300k cubic km, that's 3*10^17 kg. So you actually only need to use 0.1% of all lakes.