Compared to the fuels that the vessel is designed to carry, water might as well be liquid lead. It would easily crush the structure before you got enough in th

Compared to the fuels that the vessel is designed to carry, water might as well be liquid lead. It would easily crush the structure before you got enough in there to do hydro testing. Edit: correction, looks like they're using methane/LOX. Methane is 422g/L and LOX is 1141g/L (compared to water at 1000g/L). If it was the LOX tank then they could have probably hydro tested it. Not so much with the CH4 which is probably the upper tank? It also bears mentioning that they would want to test at the appropriate temperatures. Water isn't great for hydro testing at LOX/liquid CH4 temps.