Oceans produce small amounts of food, but substantial trade. Only ships or aircraft can enter Oceans. Landlocked Oceans are really lakes but are treated like other Oceans in all respects.
Oceans produce 1 food and 2 trade. Fish can be found in oceans which generate an extra 2 food.
With railroad (place settler on a ship in order to accomplish) the oceans produce one extra trade.
It costs 1 Movement Point to travel over an ocean region, but only ships and aircraft can do so.