The time taken by sound to travel from observer on the ship to the person on the shore $=2$ seconds.
Speed of sound in air $=320\mathrm{m}∕\mathrm{}\mathit{s}$
$\because$ Distance $=$ Speed $\times$ time
$\because$ Distance between the observer on the ship and the person on the shore
$=320\times 2$
$=640\mathrm{m}$