Final answer:
An object with a density of 0.999 g/cm3 will float in freshwater but will be almost fully submerged due to its nearly equal density to water. In salt water, with a higher density of 1027 kg/m3, the object will float better with less of it submerged.
Step-by-step explanation:
An object with a density of 0.999 g/cm3 (which is equivalent to 999 kg/m3 if we're using SI units) would either float or be suspended in freshwater, as freshwater typically has a density of about 1.000 g/cm3 or 1000 kg/m3. The buoyant force acting on the object will be almost equal to its weight if it's in freshwater. Since the object's density is slightly less than freshwater, it will be mostly submerged with just a small fraction above the surface.
However, if the object is placed in salt water, which has a higher density of around 1027 kg/m3, the buoyant force exerted by the salt water on the object would be greater than its weight, causing it to float higher in the saltwater compared to in freshwater. The fraction of the object submerged in salt water would therefore be smaller because of the increased buoyancy.
The principle determining whether an object sinks or floats is its density relative to the fluid it is in. An object with a lower density compared to the fluid will float, while an object with a higher density will sink. If the densities are very close, the object may be almost fully submerged or suspended beneath the surface.