In the environmental context, an ocean is a large body of saltwater that covers more than 70% of the Earth's surface. Oceans are crucial components of the Earth's ecosystem and play a vital role in regulating the planet's climate, water cycle, and carbon cycle.

Here are some examples of oceans:

1. Pacific Ocean: The largest and deepest ocean in the world, covering more than one-third of the Earth's surface.

2. Atlantic Ocean: The second largest ocean, stretching over 100 million square kilometers, and it's home to some of the world's busiest shipping lanes.

3. Indian Ocean: The third largest ocean, it covers around 20% of the Earth's surface, and it's known for its warm waters, diverse marine life, and coral reefs.

4. Southern Ocean: The smallest and youngest of the world's oceans, it encircles Antarctica and plays a critical role in regulating global ocean currents.

Other similar things to an ocean in the environmental context include:

1. Sea: A sea is a smaller body of saltwater that is partially enclosed by land. Seas are generally shallower than oceans, and they may have different salinity levels.

2. Gulf: A gulf is a large inlet of the sea that is partially surrounded by land. Gulfs are typically deeper than bays and may have a narrow entrance.

3. Bay: A bay is a body of water that is partially enclosed by land. Bays are usually shallower than gulfs and may have a wider entrance.

4. Estuary: An estuary is a partially enclosed body of water where freshwater from rivers and streams mixes with saltwater from the ocean. Estuaries are critical habitats for many species of fish and wildlife.

5. Lagoon: A lagoon is a shallow body of water that is separated from the ocean by a barrier, such as a reef or an island. Lagoons are typically found in tropical regions and are home to diverse ecosystems of plants and animals.