The ocean is the sea.

An ocean is one of the five very large areas of sea on the Earth's surface.

If you say that there is an ocean of something, you are emphasizing that there is a very large amount of it.

If you say that something is a drop in the ocean, you mean that it is a very small amount which is unimportant compared to the cost of other things or is so small that it has very little effect on something.