Here's what I always think. The guy in the song has been known to have zero credibility all of his life, so when he claims to see submarines and tries to warn everyone, no one believes him. It stresses him out ("overnight my hair turned grey"). But there is nothing he can do about it because "in the end it boils down to credibility" and he has none. Perhaps the submarines bring danger, and perhaps not, but either way it doesn't really matter because he will die knowing what no one else knows and what no one else will believe.
Here's what I always think. The guy in the song has been known to have zero credibility all of his life, so when he claims to see submarines and tries to warn everyone, no one believes him. It stresses him out ("overnight my hair turned grey"). But there is nothing he can do about it because "in the end it boils down to credibility" and he has none. Perhaps the submarines bring danger, and perhaps not, but either way it doesn't really matter because he will die knowing what no one else knows and what no one else will believe.