David Carlson, co-owner of Three Tides & Marshall Wharf Brewing Co. in Belfast, knows that since his waterfront businesses abruptly closed in mid-April, most people want just two things from him. One is answers. The other is beer. At this moment, both are in short supply, but he’s still willing to do what he can to solve the mystery of why one of Maine’s best-known breweries and best-loved bars is not open for business in the busy summer season. Like any good story, it’s complicated — David Carlson and his wife, Sarah Carlson, had in February announced their intention to sell, or close, Three Tides & Marshall Wharf this summer.