Again; the physics of it is that there is no such thing as negative buoyancy. Buoyant force is always in opposition to the pull of gravity; and it's regardless of the density of the item that is submerged. Salt water weighs ~8 lbs per gallon; so if you displace a volume of 1 gallon whatever is taking the place of the water will weigh ~8 lbs/gallon less in water.
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Now measure the force the block is pulling on the line and you have the force of NEGATIVE BUOYANCY!

No, you have the difference between the weight of the object and the buoyant force that acts against it. Let's say you have a 1 gallon container full of molasses; and say it weighs 10 lbs (6 lbs water 3 lbs sugar). In salt water the pull on that string would be 2 lbs (10lbs  8 lbs). The buoyant force is 8 lbs; it will always be 8 lbs for 1 gallon of displacement, whether the object floats or not is dependent on it's density relative to the surrounding water.