Is it necessary to cite information that is common knowledge or widely accessible, like historical information or popular scientific information, e. g. 70% of the earth is covered in water?

Answer: No

Explanation:

In academic writing, common knowledge refers to information found in multiple sources without a distinct origin, data that any educated person can consider valid without a source reference. For example, well-known historical dates and long-established ideas for a specific field, like key equations in physics. Observations recognized as common knowledge don't need a citation.