An opening, for example in a buoy body, to permit personnel to enter the interior for occasional inspections or repairs. It is closed by means of a firmly-secured watertight cover.
On vessels and on Lanbys, a large access manhole is called a deck hatch.
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