Somewhere I read that salt has over 14,000 uses. It is no wonder that a large number of diverse phrases using the word salt have found their way into people's language. Roman soldiers were supposedly paid in part by a ration of salt. Hence the phrase "not worth his salt". In former times, before salt and pepper shakers, a container of salt was placed on the table at mealtime. The host sat at one end of the table, the head of the table. The salt server was in the middle. Those who sat closest to the host were the most honored guests and those most important in rank. Those who sat at the far end or "below the salt" were of lesser rank and social importance. Salty language may mean one swears like a sailor. Salty in a social context may refer to social significance. In the song in question, one can substitute penis, oral sex, or vagina which is supposedly another term associated with salty dog, "Let me be your ..." and you get a phrase one would only sing if drunk or demented. Pimp would make a little more sense. "Let me be your main man or I won't be your man at all" makes a lot more sense. The phrase "salty dog" instead of main man adds a bit more punch and carries a little more sassy interpretation. But I suspect that salty dog will mean whatever people want it to mean.