A more thorough explanation:

Sea refers to:

1. The ocean
2. A large body of saltwater smaller than a regular ocean, such as the Mediterranean Sea
3. The ocean swell, which can be rough
4. An extremely large or extended quantity, such as a sea of documents

For example, when we say "on the sea," we mean on the ocean. The Mediterranean Sea is a large landlocked part of the ocean. A rough sea refers to the ocean swell that can be choppy and turbulent. A sea of documents means a large quantity of papers or files.

High seas refer to:

1. The seas or oceans beyond the jurisdiction of any country
2. Under traditional international law, the high seas began 3 miles from the coast; today, the distance is generally accepted to be 12 miles
3. Under the 1982 U.N. Convention on the Law of the Sea, coastal shores now have a 200-mile exclusive economic zone

For example, if a ship is sailing in the high seas, it is not under the jurisdiction of any country. The distance of the high seas from the coast has changed over time. Today, it is generally accepted to be 12 miles. However, coastal shores have a 200-mile exclusive economic zone according to the 1982 U.N. Convention on the Law of the Sea.