Upthrust, floating and sinking - Higher
Upthrust
An object that is partly, or completely, submerged experiences a greater pressure on its bottom surface than on its top surface. This causes a resultant force upwards. This force is called Upwards force exerted by a liquid or gas on an object floating in it..
An object in a fluid is displacing some of the fluid. The upthrust force is equal in size to the weight of the fluid displaced by the object.
Floating and sinking
If the upthrust is less than the weight of the object, the object will sink.
If the density of the object is greater than the density of the fluid, the object can never displace enough fluid to create an upthrust that will hold its weight up so it sinks.
The sinking only stops when the object reaches the bottom and an extra reaction force is added to the upthrust in order to balance the weight.
If the upthrust is larger than the weight of the object, the object will rise.
This happens when you jump into a swimming pool and then rise back up to the surface. Under the surface, the weight of water you displace - the upthrust - is more than your weight, so there is a resultant upwards force.
When your head sticks up out of the water, the upthrust reduces as your head is no longer displacing water, and there is zero resultant force, so you float at that level.