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:
Oil
Cement
Fuel
Concrete
Update: On July 23rd, 2019 new commodities were added:
Bread
Cake
Candy
Cereals
Flour
Fruit
Jam
Sugar
Sugarcane
Uniform
Vinegar
Wheat
Wine
Update: On August 13th, 2019 new commodities were added:
Bricks
Bronze
Canned Fish
Clay
Copper
Cotton
Nails
Planks
Sand
Tin
Tools
Update: On March 17th, 2020 new commodity was added:
Are a special currency, which can be acquired in limited amounts in the game, mainly as Achievements and Contracts rewards. It can also be purchased for real life money.
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.