Sunken ships from 1.13 onwards to the most recent snapshot as of this report (18w32a) seem to spawn with the wrong orientation of slabs. Instead of being upwards like if they were placed on a roof, they all spawn as the default downwards orientation.

What I expected to happen was:
Ships spawning with the correct slab orientation data

What actually happened was:
Ships spawning with slab orientations reset to default(down)

Steps to Reproduce:
1. Find any sunken ship with a captain's quarters, located at the back of the ship. It's visible there due to it being the main one with slabs.
2. You should see that the slabs are not how they should be. The ones by the entrance are most noticeable as you cannot enter without sprint swimming, and seeing the structure from the top, it's very apparent the slabs should not be that way. Most notable with the captain's table, which looks more like ┌_┐instead the intended ┌─┐.