Buoyancy

How does a fish manage to rise up and move down in the water?

A block of wood of weight $200 \mathrm{g}$ floats on the surface of the water. If the volume of block is $300 {\mathrm{cm}}^3$, calculate the upthrust due to water in terms of newton.

(Take $g=10 \mathrm{m} {\mathrm{s}}^{-2}$)

Why it is easy to swim in sea water than river water?

The magnitude of buoyant force acting on an object immersed in a liquid depends on _____ of the liquid.