Hardtack.

I think that adding dried fruit would indeed lessen the time ships biscuit could be kept. We dry our own fruit & vegetables, & it keeps well in a jar in the pantry, but I would not combine the two. I used to like biscuits with dried fruit, & I think that is still a good idea, but in my opinion you are better off using a different biscuit mix if you are adding dried fruit. Ships Biscuit was very hard, & in the 18th century it was very often soaked in a soup or similar to make it edible. I read that it was even eaten dunked in rum. Ships Biscuit is purely a survival food. It was never meant to be tasty.
Keith.