Question

What causes buoyant force (or upthrust) on a boat?

Answer 1

Archimedes principle states that a body partially or fully submerged in a liquid or gas at the state of rest is acted upon by an upward buoyant force the magnitude of which is equal to the fluid displaced by the body.
Buoyancy is the force that acts on the object when it is submerged partially or fully in a liquid.

Complete step by step answer:
When an object is immersed fully or partially in some liquid with a density $\rho$ , the object experiences an upward force that causes a net decrease in its weight. This makes the object float in the liquid.
Consider the boat is placed on the surface of water. It will experience an upward force from the liquid. This upward force will be because of the pressure applied by the liquid particles on the boat. This pressure is applied to maintain an equilibrium state.
This equilibrium state is described by balanced forces acting on the boat. These forces are:
A. Its effective weight acting downwards
B. The buoyant force acting upwards
Now we know that the Archimedes principle states that a body partially or fully submerged in a liquid or gas at the state of rest is acted upon by an upward buoyant force the magnitude of which is equal to the fluid displaced by the body.
Hence, we can say that the buoyant force experienced by the boat is due the weight of liquid it displaces from the water surface.

Note:
The buoyant force is due to the weight of the liquid displaced and not the mass of the liquid. Since the weight of the liquid depends on the effective acceleration experienced by the system, if the whole system is accelerated, then the weight of the liquid displaced would be different. This force makes an object float in water.