A watertight cover to a small opening, for example in a buoy body, through which inspection or adjustment of equipment inside may be made.
The German term Handloch is used for such a small opening.
In the U.S.A, the term pocket cover is used for the cover over the opening to the battery pocket.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM