You might have to interpret the image less literally, maybe your biggest fear is being taken to a place or situation you can't control, like the shark dragging you underwater. But I think it's more likely that your mind wasn't ready for the question, and just showed you something that's scary, not the scariest. Asking again while your mind has had time to process the request might yield different results. (that's a really manual-like sentence)

When I manage to become lucid (confidence is everything), I'll ask the same question. I wonder if my subconscious agrees with me about my biggest fear (which is losing my intellect/sanity).