Last year's first-ever fatal shark attack jolted Mainers into acknowledging that great whites regularly swim off the state's shores — and that there’s plenty about them we don’t know. Monday, July 27, 2020, dawned sultry and bright on Bailey Island, a village of about 400 full-time residents in the midcoast town of Harpswell. Early that morning, lobsterboats sputtered out of picturesque Mackerel Cove. A lone harbor seal appeared, then ducked back underwater. A handful of summer visitors watched from their docks, sipping coffee. On the cove’s east side, at a small and shallow inlet off the bridged island’s main road,...