46 CFR §170.260

1. (a)Except as provided in paragraphs (b) and (c) of this section, a Class 2 door is permitted only if—
   1. (1)Its sill is above the deepest load line; and
   2. (2)It is not a door described in § 170.265(d).
2. (b)If passenger spaces are located below the bulkhead deck, Class 2 doors with sills below the deepest load line may be used if—
   1. (1)The number of watertight doors located below the deepest load line that are used intermittently during operation of the vessel does not exceed two, and;
   2. (2)The doors provide access to or are within spaces containing machinery.
3. (c)If no passenger spaces are located below the bulkhead deck, Class 2 doors may be used if the number of watertight doors located below the deepest load line that are used intermittently during operation of the vessel does not exceed five.
4. (d)In determining whether Class 2 doors are allowed under paragraph (c) of this section, the watertight doors at the entrance to shaft tunnels need not be counted. If Class 2 doors are allowed under paragraph (c) of this section, the doors at the entrance to shaft tunnels may also be Class 2.