Hard parts of plants and animals and solid deposits are cycled to the earth's interior along with suspended sediments that have settled out of the seawater. Studies of fossils and rocks indicate that the composition of seawater has changed little over the past 600 million years. The same proportion, meaning the same chemical composition, is found in seawater of any salinity of any sample taken from any location anywhere in the world, from any depth of the ocean, or taken any time of the year. Seawater has a remarkably uniform composition that varies only in concentration. This means that the ocean is well mixed and thoroughly stirred around the entire earth.
Cold water is generally more dense than warm water, thus sinking and displacing warmer water.
Likewise, seawater with a larger amount of suspended sediments has a higher relative density than clear water, so it sinks and displaces clear water. Thus sediments, increased salinity, and decreased temperature will cause the greatest density of seawater. Most waves move toward the shore at some angle. As the wave crest nearest the shore starts to slow, the part still over deep water continues on at the same velocity. Thus, waves always appear to approach the shore head-on, arriving at the same time on all parts of the shore.