I haven't found this picture in the Library of Congress Collection, but you may look at it in the Blake Archive, and this may be a good substitute:
It's the moment when a person perceives that he's in trouble and needs help. Until that moment you are the "the master of your fate, the captain of your soul". Like the poor unfortunate in the picture the Sea of Time and Space has gotten the best of you, and you're crying for help.
Blake understood this feeling when he wrote
Children's Gates of Paradise (in 1793), but the decisive help for him came some years later (in 1800).
There is only one line ("10. In Time's Ocean falling, drown'd;") in Of the Gates, but the picture here is certainly worth a thousand words.