Murkiness is a quality of being foggy or muddy. You may note the murkiness of a pond and, fearing leeches (or sea monsters), decide not to swim with your friends.
Your sailboat might cut through the murkiness of a foggy bay, and an alligator hunter might brave a swamp's murkiness. Figuratively, murkiness can mean a state of being unclear, sometimes deliberately: "The murkiness of those back-room deals makes me very distrustful of the whole organization." Murkiness comes from the noun murk, "darkness that makes it difficult to see."