By putting Settlers on a transport unit you can build roads and Railroads on water squares. Using the „fast settler“ exploit you can easily railroad your whole 20square cross. Fish squares will produce one bonus food and one bonus trade when railroaded. Regular ocean fields will produce one extra trade only. The roads on water have no effect on ship movement. They cannot be used by land units either. A fully developped city on the coastline can almost double the trade output and will be a good Partner for a trade Route.