For this you need a magic Fishing Rod, which can be found on one of the islands
in the Greek Archipelago. You also need a bait bucket, which can be bought from
Bart, the Bartender, 2 south of Recall. With this you can dig for bait in any
room with the words 'earth', soil, 'sand', or 'mud' in the description.
Then you seek out a 'water' room, it can be sea, lake, river, shore etc. When
you've chosen a nice spot, just type FISH.
You may catch any type of fish or other water creatures, but sometimes you also
fish up debris from the bottom. Mostly this is junk, but if you are lucky, you
might hook some really valuable items.
The Fish Mongers in Olde Yorke, Crete and Dun Shivaar will buy the fish you
catch. Fishing is a bit risky, since some of the creatures you fish up might be
reasonably high level.