Why it is easier to swim in the sea than in a river?

When a ship enters a sea from a river, what happens?

An ice cube is floating in a glass of water. What happens to the water level when the ice melts? It –

A piece of ice is floating in a beaker containing water up to its brim. When whole of the ice melts

A sudden fall in barometer reading indicates that the weather will be :

Why does an iron nail float on mercury while it sinks in water?