Between what two points are you taking your continuity measurements? If you have an aluminum hull and an electric start motor, you should have some level of continuity between the hull and the negative post of your starting battery. If you are connecting your accessory to a battery other than the one that the outboard is connected to, then the accessory is the point of ground to the hull for that battery. Also keep in mind that if the factory ground straps on the outboard have a high resistance connection for whatever reason, when the boat is placed in the water, continuity will exist between the bare areas of the grounded outboard and unpainted surfaces of the boat that are below the water surface. A stainless steel screw in the transom is an example of a common point of the circuit bridge. Keep in mind that the paint is an insulator. You will also find that the steering cable and helm will be grounded and complete the circuit back to the starting battery.