When you swim below the surface of the water, you don't feel the weight of the water above you because of the concept of buoyancy. Buoyancy is the upward force exerted on an object immersed in a fluid, such as water, which opposes the force of gravity.
When you are submerged in water, the pressure increases with depth due to the weight of the water above you. However, the pressure is exerted equally in all directions, including upward. This upward pressure counteracts the force of gravity and creates an upward buoyant force.
The magnitude of the buoyant force depends on the volume of water displaced by your body. Archimedes' principle states that the buoyant force acting on an object immersed in a fluid is equal to the weight of the fluid displaced by the object. Since your body displaces an amount of water equal to its volume, the buoyant force acting on you is equal to the weight of that displaced water.
Because the buoyant force and the force of gravity are balanced, you don't feel the weight of the water above you when swimming below the surface. Instead, you experience a sensation of weightlessness or reduced weight due to the buoyant force counteracting gravity. This is why swimming can feel lighter and less strenuous compared to activities on land.