You can use a Fishing Rod to gather Fish at any body of water and sell it to the Market.
To gather Fish simply go to the water's edge with your Fishing Rod equipped, now you should see the option "Catch Fish".
You can also buy your Dive Gear from the Dive Shop, dive into the water and swim up to a fish and click "Catch Fish".
If you want to go pro and earn more money from fishing then go gather FreshFish at any of the fishing areas at sea. Bring the FreshFish to the Factory to process them into Fish, which you can then sell at the Market.
For fishing at sea, you require a boat licence and a boat. Get to one of the fishing area's at sea to catch fish.
The Blue fishing zone has small fish, the Green has sharks and the Red has large sharks.
Shark meat is worth more than fish.
Note: When using a Fishing boat you'll need to drop the booms and use the option "cast net". Then hold Q and drive between 5 and 15. You will see a message that says "Fishing".
Note: Be careful not to become over-encumbered!