* San Francisco Bay: A specific body of water in California.
* Hudson Bay: A large body of water in Canada.
* Chesapeake Bay: A large estuary on the Atlantic coast of the US.
* Bay of Bengal: A large bay in the Indian Ocean.
Key points to remember about proper nouns:
* They are specific names of people, places, or things.
* They are always capitalized.
* They are not generic terms.
So, while "bay" is a common noun referring to a general type of body of water, "San Francisco Bay" is a specific bay, and therefore a proper noun.