President Trump can’t hold his large energetic rallies because of coronavirus. But his campaign has discovered the next best thing: massive pro-Trump flotillas that are taking his message to waterways across the country. The large boat parades — which began organically among MAGA devotees in South Florida and spread to Florida’s Gulf Coast, Arizona’s Lake Havasu, South Carolina’s Charleston Harbor and Southern California’s Newport Harbor during Memorial Day weekend — quickly caught the notice of the president and his campaign. Now, the campaign is encouraging the flotillas and utilizing its robust data operation to organize and excite a demographic that...