46 CFR §170.255

1. (a)Except as provided in paragraphs (b) and (c) of this section, Class 1 doors within passenger, crew, and working spaces are permitted only above a deck, the molded line of which, at its lowest point at side, is at least 7 feet (2.14 meters) above the deepest load line.
2. (b)Class 1 doors are permitted within passenger, crew, and working spaces, wherever located, if—
   1. (1)In the judgment of the OCMI, the door is in a location where it will be closed at all times except when actually in use; and
   2. (2)The vessel is less than 150 gross tons and will not proceed more than 20 nautical miles (37 kilometers) from shore; or
   3. (3)The vessel is in rivers or lakes, bays, and sounds service.
3. (c)Class 1 doors are permitted in any location on a vessel that—
   1. (1)Is less than 100 gross tons; and
   2. (2)Will operate only in the offshore oil industry trade.
4. (d)Quick-acting Class 1 doors are permitted in any location on a vessel that operates on the Great Lakes and is required to meet the damage stability standards of subpart H of part 172 of this chapter.
5. (e)For vessels required to meet the damage stability standards of subpart H of this chapter, when Class 1 doors are installed below a deck the molded line of which at its lowest point at side is less than 7 feet (2.14 meters) above the deepest load line, an indicator light for each door which warns when the door is open must be installed on the bridge.