Worth one's salt
To be effective and efficient; deserving of one's pay.
Sodium chloride, a.k.a. salt, is essential for human life and, until the invention of canning and refrigeration, was the primary method of preservation of food. Not surprisingly, it has long been considered valuable.
To be 'worth one's salt' is to be worth one's pay. Our word salary derives from the Latin salarium, (sal is the Latin word for salt). There is some debate over the origin of the word salarium, but most scholars accept that it was the money allowed to Roman soldiers for the purchase of salt. Roman soldiers weren't actually paid in salt, as some suggest. They were obliged to buy their own food, weapons etc. and had the cost of these deducted from their wages in advance.
Salt continues to be important enough to feature in the language for many centuries. Other phrases that would have been known to the mediaeval mind were take with a grain of salt, the salt of the earth and below the salt. The ancient roots of 'worth one's salt', and its similarity to the 13th century 'worth one's weight in gold' and the 14th century 'worth one's while' (that is, worth one's time), give the phrase a historical air. Nevertheless, 'worth one's salt' didn't exist in Roman Latin or even in mediaeval English and dates from as recently as the 19th century.
The earliest citation of the phrase that I have found in print is in The African Memoranda, a report of an expedition to Guinea Bissau, by Philip Beaver, 1805:
"Hayles has been my most useful man, but of late not worth his salt."
It's worth pointing out that, although English is replete with phrases of a nautical origin, none of the above salty phrases has anything to do with the sea.