turtle-deck

We stood on a little circular space of the turtle-deck, well aft -- an observation space enclosed by a low metal rail.

Tarrano the Conqueror Ray Cummings 1922

There was not a soul standing around on the main deck of the destroyer as we passed her, nor on her high forward turtle-deck, which was being washed clean; and surely not much comfort being bounced around on transoms in that destroyer below, nor too much dryness on her flying bridge.

The U-boat hunters 1912