Of all the spirituals I’ve sung with my own choir, this one, with its slow pace and minor key, conveys the feelings of an oppressed people the most strongly. It’s not just a series of complaints or calls for help, though. There’s a lot of scriptural truth packed into it.
Let me start out with the meaning of “rock.” As in “Elijah Rock,” the overall reference is to God, but this song spells out a couple of specific ones. God is “a rock in a weary land.” You might think that the word “weary” was put there by the slaves who sang it, but it’s straight from the book of Isaiah in the old King James Version: “The shadow of a rock in a weary land.” Other translations substitute the word “desert” or “parched” for “weary.” So the original meaning probably doesn’t have anything to do with actual physical weariness, but that aspect must have appealed to people whose lives were one great stretch of it. The idea of shade and rest is implicit in the text, as is that of protection: “a shelter in the time of storm” is also from Isaiah. It’s not at all uncommon, by the way, for us to say, “He’s my rock” to refer to a person in our lives who keeps us on track and is always dependable
The simplified explanation of how black spirituals came about goes like this: slaves heard about Christianity after arriving in the US and, especially on the southern plantations, came up with sung versions of those teachings that gave them hope of a better life, expressed their longings for deliverance, and often served as rhythmic work songs. All of this is perfectly true but raises further questions: how did this “hearing about Christianity” come about? And why, if you think about it, would slaves adopt the religion of those who had enslaved them? Wouldn’t they see that religion as massive hypocrisy on the part of the slave owners? There’s also a tendency, which I have unfortunately shared, to think of spirituals as rather primitive. But that tendency is clearly mistaken: these are songs with deep meaning, displaying a breadth of Scriptural knowledge. To quote a modern African-American scholar and preacher, 
Like many other Christmas carols, the words to “I Saw Three Ships” don’t make much sense when you look closely at them. Had you ever asked yourself the question in the title? Probably not. Most of us don’t carry around a map of Palestine in our heads, so the absurdities of the lyrics aren’t obvious. I’m therefore including such a map below this post, which will serve to prove that indeed it would be impossible for the words of this carol to have any connection to reality at all. So what on earth are the words talking about?