This dirty water is more than likely a wrong or unclean spirit. Numbers are very significant in the Word of God as well as in dreams. The number seven is a type of perfection and completion. Seven is one of God's perfect numbers. It is used about 600 times throughout the Bible.
Colors are also important to pay attention to in a dream.
Many times a color will stand out so vividly in a dream that you just know it is important to its meaning. I would not suggest taking every color from a dream and picking it apart. However, if something stands out as significant, pay close attention to it. An example of this would be a dream that my husband had. In the dream the streets of a downtown portion of the town we live in was full of water.