I've tried looking up deadnails in various dictionaries/encyclopedia etc. all to no avail. But I seem to recall that a deadnail is a particular type of nail, maybe one without a head? And I have also heard "lamproom floor" sung instead of "door", which leads me to speculate wether nails with no heads were used on the floor, so as to reduce the risk of sparks (caused by sailors bare feet skating across the nails!!!)
Dead Horse was often served up as beef aboard ship, provided by unscrupulous suppliers. A barrel of a certain size was also called a "horse cask" and these were used to contain beef, water, etc. aboard ship.
And if you think this ain't not true
And they say so, and they hope so
In the horse cask look, you'll find his shoe
Oh, poor old horse.