The correct answer is C) Opening a sideport at sea to renew a gasket. The Official Logbook must record any openings of sideports or other openings in the vessel's hull while at sea, as this is considered a significant event that could impact the vessel's seaworthiness and safety. Entries like the biennial weight test of lifeboats (A), annual lifeboat maintenance (B), or recording the vessel's drafts on entering port (D) are also important, but are not specifically required to be entered into the Official Logbook. The logbook is a legal document, and recording events like hull openings is necessary to document the vessel's operational history and compliance with regulations.