During the twenty years I lived in Buenos Aires I always enjoyed fishing in the Parana Delta during the warm season (October to April) I visited a number of fishing places along the Río Capitan (La Fusta, Paso del Toro) and along the Paraná de las Palmas (Crovetto, Laura, Boca Toledo, Yacaré, amongst others). I seldom went further than that. Travel on a motorboat to the Parana de las Palmas can take anything between an hour and an hour and a half each way, depending on whether the river is low or high, since sea tides have an influence on the level of the River Plate. I have read of captures of dorados nearer the Parana Guazú and beyond (recreo Martinez), but personally was never able to fish a dorado here, although once I saw one jumping for food. Basically I fished bogas (a sort of small mouth bass), and fishing them was real fun. They start sucking the bait, and if you are too anxious to catch them you will not fish any. And they might even try to take the bait around a pillar of the pier, and if they succeed, you will loose the fish. Its mouth is very week, so you have to be very careful to take the fish out of the water gently. If the fish is large enough (over 4 pounds, because it has many spines) grilled with lemon juice it is delicious. But touring the Delta or Tigre just for the joy of touring it, is really worthwhile.
You can even have lunch on a catamaran, but I discourage using anything else than regular motorboat transportation service (Interisleña), because catamarans need more depth for navigating, and miss some of the most beautiful places. A round trip will cost less than 5 dollars and it is an enjoyable experience. You can get off practically anywhere, but will be expected either to pay an admission fee (maybe 1 or 2 dollars) or to eat a sandwich or drink something at the resort. There are also a number of hotels and resorts in this area. Some are expensive (Laura, 3 stars) and others (Boca Toledo, Yacaré) are not. There is not much walking around to do, so either you enjoy fishing, or just relax and get a nice suntan in your bathing suit. The round ticket is valid for returning on any motorboat, so you decide how long you stay.
The Parana delta is one of the largest in the world. Its surface is 14,000 square kilometers (three million and a half acres). It is 200 miles long and probably 30 miles wide, has a surface of and has hundreds of small islands connected by rivers, most of which are navigable on small motorboats. Mosquitoes could be a problem (not always) so take Off repellent, just in case. I have really enjoyed every single day in the Paraná Delta (also known as Tigre) and I am sure you will also. Trains from Retiro station (round trip costs only 70 cents of a dollar) run every 10 to 15 minutes to Tigre. Travel takes nearly one hour. But 60 also comes here from Plaza Constitución. Whether you travel on the train or on the bus, you will go through beautiful residential areas with lovely residences and parks (Olivos, La Lucila, Martinez, Acassuso). You will really enjoy the trip. You can get off at any station, walk around and return to the train (but you will have to buy another ticket). There is no need of buying a local tour for visiting this area. I certainly would buy the Buenos Aires city tour, because otherwise you would miss very interesting spots. But do that another day. Have a nice time in Buenos Aires.