Season, weather and time of day are as important as location when it comes to best places to fish for money.
Assuming that the player has sufficient skill to catch whatever takes the bait, my feeling is that the best place for a day's fishing is probably at the beach on a sunny day in Summer. Throughout the whole day there's no fish with a base price below 75g (Red Mullet and Tilapia, plus Halibut at 80g), there's the 100g fish (Tuna, Flounder) and then there's the pretty valuable stuff like Octopus at 150g, Pufferfish at 200g and Super Cucumber at 250g. With quality and profession multipliers, with wild bait to increase the bite rate, and with mostly iridium quality fish, with gold quality on the top end stuff because they are more difficult, 15,000g per day is very much within reach.