Understanding a watershed is tough. I think it is because the name does not fit the definition. A watershed is an area of land that drains into a shared body of water. It could be a shared stream, lake, river, estuary, aquifer, or ocean. What links the land to these water bodies are wetlands.
Each watershed is connected to another watershed. All watersheds eventually lead to the ocean. In the U.S. there are over 2,200 watersheds (including Puerto Rico). They have no restrictions on counties, state, or even national border lines.
All watersheds are delicate and their strength is dependent upon 1) what we put in them and 2) how we use them. Follow how I am learning to make less on an impact on my watershed.