Kenny Mann, a Sag Harbor filmmaker whose feature “Beautiful Tree, Severed Roots” was shown at the Hamptons Take 2 Documentary Film Festival in 2013, has written and produced “The Mtepe Shungwaya Sails Again: A Tribute to the Boat-Builders of Lamu,” a new 13-minute documentary that will be shown at the Maritime Museum of San Diego on Friday, Oct. 2.