Enormous amount of water evaporates from the sea in an year. This should have gradually dried up the seas. But the nature’s balance is so perfect that which every season, all the water that is drawn up into the sky returns to the sea in different ways.
This maintains the same level of water in the seas despite continuous evaporation. Rainfall, snow all ultimately reach the sea maintaining the lost balance.
As they fall in rivers, springs and streams, they ultimately flow back into the sea. Thus, the continuous water cycle never lets the oceans and seas dry up.
Below Sea info from Wikipedia (Read More):-
The term sea refers to certain large amounts of water, but there is inconsistency as to its precise definition and application. Most commonly, a sea may refer to a large expanse of saline water connected with an ocean, but it is also used sometimes for a large saline lake that lacks a natural outlet, e.g. the Caspian Sea. Colloquially, the term is used as a synonym for ocean. Additionally, large lakes, such as the Great Lakes of North America, are occasionally referred to as "inland seas".
http://en.wikipedia.org/wiki/Sea
