Look up salty dog in Wiktionary, the free dictionary.
“Salty Dog” is nautical slang for an experienced sailor who has spent much of his life aboard a ship at sea. A salty dog is often given increased credibility by ship mates in matters pertaining to ship-board life and duties. Also known as an “old salt.” 
Another meaning of the term "Salty Dog" comes from the archaic practice of rubbing salt into the coat of one's favorite dog as a flea repellent. Therefore, one's "salty dog" is one's favorite person, best friend, etc... This is the meaning of the line in "Salty Dog Blues": "Let me be your salty dog, or I don't wanna be your man at all." The phrase could also have a sexual meaning. The lyric "Honey, let me be your salty dog" could also be translated to mean "Let me be your sexual partner."