Fish are the only Natural Resource that is found in water, and are used as a source of Food. Fish are currently not a sustainable food source although they are relatively abundant, especially in the Lake biome. Fish provide twice the nutritional value to mushrooms, and half the value of rabbit meat.
Fish are hidden until discovered through the act of fishing. Fishing within a radius of a Fish tile has a chance to catch them (which moves the tile to the fishing spot); fishing closer to the tile has a greater chance of yield. After a few catches a fish tile will run out, which means you will need to move on to deeper waters to find more.