The Nahant harbormaster and assistant harbormasters have been warning away power boaters who come too close to swimmers on local beaches, and plan to issue citations in the future, said harbormaster Rob Tibbo. “There is no such thing as a minor injury from a propeller,” said Tibbo, who noted that power boats by law are allowed no closer than 150 feet from shore. While the majority of boaters obey the rules, Tibbo said he knows of some boaters who have been warned to stay away by him and assistant harbormasters, but come close to shore again when the harbormasters’ boat leaves the area.