You need to research Farm tech to learn how to fish -- if you don't have the appropriate tech level, your farmers will never fish no matter how many times you put them in the ocean.

You also need to make sure they have a nice clean area to fish from, too.

There are a bunch of other threads about fishing if you need more information, or check out this forum for a hints/spoilers/strategies.

good luck!
-- jen
_________________________
It is pitch black. You are likely to be eaten by a grue.