Fishing's great. If you can, learn the Fisher skill at level 5, you can sell fish at a better price. Max skill most people go Pirate for the added chance to find treasure, but Angler works well for moneymaking (treasure is usually for utility, not sell for profit). Fish smoker is a great addition to the game, if you throw rare/expensive fish in and with higher quality star indication. Fish ponds used to be great moneymakers in 1.5 but i see here they have somewhat reduced productivity (might be wrong, am still inspecting), although people report the endgame item Golden Crackers turn one into a "goldmine".
If you like fishing and aren't far into the game... You should play the riverlands farm or even better, the beach farm, chosen at the beginning of the game.
And don't forget once unlocking cooking, to turn the cheaper crab pot items yielded into cooked dishes and fertilizers. As soon as you can, unlock the iridium rod and you should be fine!