Floating and sinking are often the subject of study in science lessons. An object that is afloat is an object in equilibrium. To keep an object afloat there must be an upward force to balance the gravity force acting on the object. You can feel this upward force whenever you swim or try to submerge yourself in the bath. (This buoyancy force is sometimes called the upthrust.)
This buoyancy force is shown by a vertical arrow from the bottom of the object, since the origin of the force depends on interactions between the object and the particles bombarding its surface.
To explain the origin of this buoyancy force (which provides an upthrust) you need to think about the pressure in the surrounding fluid (liquid or gas), and the resultant forces acting on the surfaces of the object by the fluid environment. You'll also see why we chose to draw the force arrow from the bottom surface.
Expect buoyancy forces wherever one object is in contact with its surroundings in such a way that there will be particles of the fluid bouncing off the object's surface.