Now we're starting to get a bit bushy, crafty and even (maybe) survivally!
Saltines are a modern version of hardtack, which is more or less the same as ship's biscuit.
http://en.wikipedia.org/wiki/Hardtack
http://en.wikipedia.org/wiki/Saltine_cracker
The word "biscuit" is borrowed from French, and means "twice-cooked", originally to drive off more moisture in order to prolong its useful shelf-life (thought it wasn't called that back then)... The equivalent German term "Zweiback" also means "twice baked" (though the Germans have also borrowed "Biskuit").
The sailors of Brittany used to take on their ships loaves of bread that had been baked a second time; this is still known in some ports and is called "pain recuit" in French. I've forgotten its name in Breton... "bara" is bread, "daou" is two, but I'm having trouble finding the proper term for "twice-baked"...
I found a recipe for Nelson's ship's biscuits, but in French, in Picky Weedier.
http://fr.wikipedia.org/wiki/Biscuit_de_mer#Red.C3.A9couvrir