Each week when the massive LNG tanker invades the Mystic River with machine gun-toting Coast Guard boats, state police helicopters thumping overhead and divers in the dirty water searching piers for explosives, nobody bats an eye. ‘‘Where we gonna go if it blows?’’ says John Harrington III, shrugging behind the field's backstop. ‘‘What are you gonna do if it goes? Where can you run to? You can just kiss your ass good-bye.’’