I know an old salt is an old sailor in maritime jargon, but where does the term originate. Does it have to do with the fact that sea water is salty? Why does the old salt have to be old, can’t s/he be just a salt (not assault, that’d be awful)? Or sea salt?
Apparently it refers to the salty water of the seas, according to Etymonline. Old refers to veteran, experienced:
Meaning "experienced sailor" is first attested 1840, in reference to the salinity of the sea.
Green’s Dictionary of Slang gives usage examples from 1830:
1830: N. Ames Mariner’s Sketches 7: The ceremony of shaving on crossing the line was omitted, to the manifest disappointment of the ‘old salts’.
Conventionally old salt would have been literally what it sounded like. Searches of both Early English Books Online (EEBO) and Eighteenth Century Collections Online (ECCO) yield a lot of collocations like "old salt butter" (for cooking) or "old salt beef" (for military ration) and "old salt works/pits" in addition to "old salt" itself. Similar collocations are visible in a Google Books search between 1800 and 1830.
I can find no origin point for the sailor term earlier than this entry in the Oxford English Dictionary in "old salt, n." within "old, adj.":
1828 Ariel (Philadelphia) 23 Aug. 72/1 Landsmen generally have very mistaken notions concerning sailors... ‘Tom Coffin’ is a caricature, and not a very good one of an ‘old salt’, but terribly strained and stiff.
So why "old salt?" The sailor would be old, and sailors would be associated with the salty brine. Furthermore, there were existing associations between "old salt" and the quality of food preparation. For instance, one 1816 magazine claims:
Old salt (clean muriate of soda) is regarded as the best, and it is used grated or pounded. New salt being generally deliquescent is less adapted for salting ...
Old salt is a better curer because it doesn't turn liquid as easily from moisture in the air (that's what deliquescent means anyway). Furthermore, corned beef or, as some military books called it, old salted beef, was a common ration for both army and naval units. Here's William Blair writing The Soldier's Friend: or, the means of preserving the health of military men; addressed to the officers of the British army in 1798, accessed behind the paywalled ECCO:
Where either salt-fish or salmon is used, however, it should be boiled in sea-water, which not only saves the expense of salt, but also renders the food more agreeable; even very old salt beef is improved, and rendered more palatable, by first steeping, and afterward boiling, it in salt water.
There's no evidence for a direct connection between old salt and rations, but the shared context between the two suggests an association between the oldest and most ragged of rations and an old, stringy, preserved sailor.