What is neutral buoyancy?

Buoyancy is the net upward force experienced by an object in a fluid. Pascal's principle dictates that fluid pressure on an object increases with depth, so there is greater pressure on the bottom of the object than the top, resulting in a net upward force. When an object's buoyancy is greater than its weight, the object will float.

An object with neutral buoyancy has a density such that when it is fully submerged the upward force due to buoyancy is exactly equal to the downward force due to the weight of the object.

In pure water this is a density of 1000 kg/m3 (1kg/litre)

In sea water this is a density of about 1025 kg/m3 (1.025kg/litre)

Consider a submarine. A sub with negative buoyancy would tend to sink . Positive buoyancy would cause it to sink. A sub with neutral buoyancy would remain at a steady depth.