Sentence:  According to Robert Hulse, 'It's half worm, half mollusc, and there are more dead men at the bottom of the sea as a result of those things <small>[shipworms]</small> than all the naval battles put together. They're the reason you had <WSD>copper-bottomed</WSD> ships'. 
Meaning of the word marked with <WSD> tags:  Having lower parts made of or covered by copper (especially of a ship or cookware).