Access to and escape from hull spaces of merchant ships as per IMO/SOLAS are simple. Same principles should be used by any Navy. The non-military principles are:
Cargo spaces have separate, weather tight hatches on comings on the weather deck for cargo and crew. Crew uses vertical ladders. Access is only from open weather decks. It is very safe and simple, if the hatches are weather tight. Before closing hatches check for stow-a-ways, etc. That hatches are weather tight are easily checked by hose tests.
.
Engine spaces are protected by a coaming with doors above weather deck. Engine crew members take a ladder up to the door. A second escape from the engine room is a vertical ladder in a trunk with a door at the bottom and a hatch on the open deck. Of course there are other ways in/out of the E.R. via doors in the deck house and the funnel for normal use. All accesses are gas tight to enable fire fighting.
Hull accommodation spaces have enclosed stairwell trunks with doors at every deck level as main access/escape. The stairwell starts inside a protected superstructure or a deckhouse. Vertical/sloping ladder in an enclosed trunk is the second escape from such hull space. Watertight doors are not recognized as regulatory escapes from hull spaces.
Above simple principles could easily be applied by Navies. I know they are not. The people involved are not very bright and more interested in shiny uniforms.