sitting duck - think of a duck sitting on a pond about to be shot Now extend that image to the story and you have it!

'Sitting duck' is an idiom that comes from hunting and means that a duck which is sitting in the water or on the ground is an easier target than one which is flying. We use it to describe someone or something which is vulnerable to attack and unlikely to put up a defence. In your sentence it means the ship was very vulnerable to attack where she was. When referring to people 'attack' is not necessarily physical but can be verbal or it means someone who is easily tricked, swindled or cheated.

sitting duck = an easy target or victim.

It means she was there, just waiting, unprotected.. nothing could stop John from getting to her.