Reversing the district court dismissal of a seaman's admiralty claims against a vessel in rem and issuing a writ of mandamus to the district court to award him maintenance in a case relating to injuries he incurred when a vessel he was working on exploded because the court had jurisdiction to review interlocutory orders dismissing the claims against the vessel as it effected his substantive rights, but lacked jurisdiction to review the denial of a summary judgment motion, though it treated the notice of appeal as a mandamus motion.