"The wrongful death claimants therefore sustained their burden of proving unseaworthiness and there remained only the issue of whether or not one or more of the conditions of unseaworthiness or some other agency caused the disaster. The court found in explicit terms that 'no one knows how the ship was lost.' The resolution of the question of liability will, under the circumstances, be determined by the allocation of the burden of proof on the causation issue, the existence of a rebuttable presumption and whether or not that presumption has been met."