A ship ladder (AKA, unfortunately, 'ships ladder' [sic]), is a very steep type of staircase; as the name suggests, they are almost closer to ladders than stairs.
Space is limited onboard a ship, and ladders are not uncommon. However, in larger yachts and many ships there is room for some basic conveniences -- barely. Ship ladders are the minimum that stairs need to be in order to ascend without the use of your hands to help climb. This usually means about a 60-70 degree incline, although some shorter ladders approach 90 degrees. The one factor that universally sets them apart from ladders is that they have treads rather than rungs.
To save space, each tread is partially recessed under the one above; there are no risers. This allows the staircase to take up much less space at the expense of increasing the risk of trips and falls. Naturally, these staircases generally have railings.
There are variants on the standard ship ladder, the most common being the samba tread ladder, in which the treads have alternating cutouts, one tread with the left side cut out, the next one up with the right side cut out. This allows for a more vertical angle, but is extraordinarily dangerous if you forget what kind of staircase you are dealing with.
Brevity Quest 2016