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?

Generally, information that is considered common knowledge or widely known does not need to be cited. Common knowledge refers to information that is widely known and accepted, and it can include facts that are available in numerous sources and are part of the general understanding in a particular field or among the general public.

In your example, the statement "70% of the earth is covered in water" is a widely known and accepted fact, so it may not require a citation. However, it's important to note that what constitutes common knowledge can vary, and if there is any doubt about whether a piece of information is widely known, it's advisable to provide a citation to be thorough and transparent.

In academic and professional writing, it's always a good practice to cite sources for specific facts, data, or ideas that are not considered common knowledge, or when there is a potential for readers to question the accuracy or origin of the information. Additionally, if you're unsure whether information is common knowledge in a specific context, it's better to err on the side of caution and provide a citation.