Why is the ocean salty?
All water is recycled. Water that falls as rain is heated by the sun and becomes water vapor, later to fall as rain again. As water runs to the sea, it carries dissolved material with it. When water evaporates, though, it leaves behind anything that was dissolved in it. (That's why rain isn't salty.) In this way, over millions of years, salt became concentrated in the oceans. Undersea volcanoes and vents may also contribute significant amounts of salt.
What causes ocean waves?
Most waves are the result of wind. Their height depends on wind speed, and how long and how far the wind blows. Then why are there waves at the beach on days when there isn't any wind? Waves may begin far out at sea. After the wind stops, the waves keep going. (Think of what happens when you drop a pebble in a still pond: The ripples keep going for a long time.)
How did all the water get in the ocean in the first place?
No one knows for sure. One popular theory is that as Earth was forming, hydrogen and oxygen gas in the atmosphere reacted to form water vapor. The vapor then fell as rain that filled up depressions on Earth's surface, forming oceans - oceans that used to be nonsalty.