Water not only can keep a ship afloat, but can also sink it. 
Sometimes, something that is useful or essential to us is also dangerous. There are times that we rely on something or someone to carry us through situations, but an overreliance on that person or thing may be our downfall.
Understanding the dangers of relying too much on someone or something can only help us. We need to be able to do things independently (this is not to say that we can't depend on others). We need to be able to do things without help if necessary. At the same time, though, we also need to be able to rely on others or objects to carry us through when we most need support.
