In around 1800, Manhattan Island was home to several merchant seaports. Some of the most important seaports in Manhattan at that time were:

1. The Port of New York: The Port of New York was the largest seaport in Manhattan and one of the busiest ports in the United States in the 1800s. It was located in the southern part of Manhattan, near the Battery, and served as a gateway for international trade.

2. South Street Seaport: South Street Seaport was another important seaport in Manhattan in the 1800s. It was located on the East River, between the Brooklyn Bridge and the Manhattan Bridge, and was a hub for maritime trade and commerce.

3. Chelsea Piers: Chelsea Piers was a series of piers on the Hudson River, located in the Chelsea neighborhood of Manhattan. In the 1800s, it was a major seaport for cargo and passenger ships.

4. Fulton Fish Market: Fulton Fish Market was a historic fish market located in Lower Manhattan, near the South Street Seaport. It was one of the largest wholesale fish markets in the world in the 1800s and played a key role in the local economy.

These seaports were important hubs for trade, transportation, and commerce in Manhattan in the 1800s, and helped to shape the city's economy and culture.