The Coast Guard has no affirmative obligation to save sailors from sinking ships. But Aaron Greenberg’s estate — relying on the Suits in Admiralty Act and the voluntary-undertaking doctrine — contends he drowned because the maritime agency allegedly blundered when it responded to an emergency radio beacon from a capsized fishing vessel.Section 323 of the Restatement (Second) of Torts says: “One who undertakes, gratuitously or for consideration, to render services to another which he should recognize as necessary for the …