If you're smart enough, and lucky, you can make your fortune fishing. You can also fish up treasure, like cannons from sunk ships.
Before you start fishing, you're going to need bait of course. To get the cheapest bait, its best to buy it straight from Daigard, where the bait mine is. The best types of bait to use are lures and ballyhoo's currently, preferably ballyhoo's if you have the money, because they can be sold fastest and for the most profit. Anything higher is too risky, and will take too long to sell for a profit. For more on bait, and some info on fish too, follow the link here.
Now what do you get when you go fishing? FISH! When you have bait in your cargo as a fisherman, you'll have a chance at regular intervals along the route to catch a fish. This means don't carry to much bait on board, or not all of it will be used. The fish will always be the same color as the bait you were using, although the type will be random. For more on fish, and some info on bait too, follow the link here.
While fishing, you may not always catch fish. It's also possible to pull up treasure from sunken ships. For example, if someone happened to have their ship sunk with an Oil Painting on it, and you pass over the ship with some bait, a piece of your bait will be replaced with the Oil Painting. Bait will only be able to acquire treasure that is its color is higher, or the same as the treasure.
While fishing, you can't just send out a yellow raft and hope to get to the next city safely, you have to take precautions. One of the easiest and most popular methods of avoiding being plundered is to use the bubbling technique. Bubbling is simply sending out multiple ships/vehicles, with the one in the middle carrying the cargo. This means the pirate will have to send out multiple ships to plunder you. Another way to avoid pirates is to use fast ships, so only ships that are faster or just as fast as your can plunder you. That means using a Ferry would be a lot better than using a Raft. For more on ships and ship speeds, follow the link here.