Buoyancy refers to the upward force exerted on an object immersed within a fluid, while hydrodynamics focuses on the motion of the fluid and its interactions with solid objects.

Buoyancy forces influence not only the local development of temperature gradients, but also the emergence of large‐scale flow patterns that are of vital importance in contexts ranging from ocean ...

If these two forces are equal, then the object will be neutrally buoyant and neither rise nor sink. It's like hovering, but in water, and it is essentially what you want to do when scuba diving.

If the buoyant force of a fluid is greater than the weight of an object placed within it, the object will float. Helium, being lighter than the other elements in our atmosphere, rises.