According to the Oxford English Dictionary, the noun main (sometimes spelled maine) was also formerly used in poetry to mean "a broad expanse." This meaning makes more sense than ocean in this case, but I'm left wondering if this song really saying "a message to the main."
That's what the lyrics sites say, but those sites are full of mistakes. I just don't hear "to the main" in this song, and remember we are going back to the 1600s to find this definition. I think it sounds like "a message to the main goal."
This doesn't seem quite right either, but I suppose it could mean "a message about the main goal," since expressions like "speak to that" seem to be gaining favor vs. "speak about that." Listen closely and see what you think; it's a great song in any case!