It gets better. Deuterium is .0156% of the hydrogen is seawater, yet is the usable portion for fusion. If seawater was 100% heavy water (water using deuterium rather than hydrogen1) it would take what? A teaspoonful to match those 40 cars?
And if the heavy water were to be electrolyzed into oxygen and deuterium and then the deuterium liquified it might take a drop?