Skilled fishermen feed the masses
The most common fishermen pull up large numbers of fish, capable of being fileted into steaks and barbequed by cooks. Some of the more seasoned anglers venture out to sea in boats, in search of great fame and riches.
To fish you will need a fishing pole equipped, use it and target a nearby body of water. You must remain close by while your cast is in the water. Occasionally, a location or tile will become overfished and you will be required to move to another spot.
To train in fishing, it will take some time and you will need to move around a great deal. Most choose to do this aboard a ship, for the ease of moving from place to place and for access to a secure hull to store your catch. You can train fishing from 0 to GM without leaving the shore line, and without fishing in a special location or manner.
At 65 fishing you can fish up sunken treasure. At GM you can fish up Message in a Bottles.