That passage is very powerful because she is saying that she was quite close to her destiny without being able to know it, just as a ship might be close to land without being able to see it. Her destiny was to meet her teacher, Anne Sullivan. This quote is from the day that Miss Sullivan arrives. Helen is aware that something is about to happen, but she does not know what it is. The entire house is buzzing and preparing and she is anxious because she cannot know what is about to happen or how it will change her life.
"Have you ever been at sea in a dense fog, when it seemed as if a tangible white darkness shut you in, and the great ship, tense and anxious, groped her way toward the shore with plummet and sounding-line, and you waited with beating heart for something to happen? I was like that ship before my education began, only I was without compass or sounding-line, and had no way of knowing how near the harbour was. "Light! give me light!" was the wordless cry of my soul, and the light of love shone on me in that very hour."
Here she is expressing that she was hopeless prior to her education. She was unable to effectively communicate with her family and they with her. So, she was very much a ship in dense fog, unable to see anything and know what was going on.