One of the key components in Seaport are materials. Regular materials, such as coins, fish and wood, can be produced in production buildings and gathered at destinations. The amount you have is displayed at the top of your screen and you can store an infinite amount of them. They are used to upgrade your port, purchase ships, and fulfill contracts.
Apart from the main materials, the game has commodities, which are stored in the Warehouses. They can be exchanged for regular materials on the Market Town destination.
Update: On November 20th, 2018 new commodities were added:
Update: On July 23rd, 2019 new commodities were added:
Update: On August 13th, 2019 new commodities were added:
Can be acquired mainly from the Fishing Pier in your port. There is also destinations to which you can send ships to get more of them. Fish are not required to purchase anything by themselves except for some buildings, but you need them to produce Wood, Stone, Iron and Mechanical Part.