Some of these swimmers went a little further and swam four times each week, about 45-minutes per swim. They got better, too. I don't have any statistical data, but my take on it was that as the swimmers added a workout each week, they got better. Three swim workouts each week was better than two workouts, and four swim practices was better than three swims. What about doing five or six workouts - or even more swimming - each week?
Elite or Olympic swimmers practice one to three times a day, 6 to 7 days each week. These are not all in-water, swimming workouts, but they are probably swimming at least once every day they have a workout. Most of us do not have that level of skill or that much time to workout every day - that's OK, we can still get a lot of benefit out of less swimming!
Most of the swimmers I work with can maintain a good level of fitness at three workouts each week, and most even improve their swimming. They often have a bigger magnitude of improvement if they swim four to five times each week, but many of them don't have time to do that many workouts. I believe that three swims per week is the minimum number of swim workouts needed to feel some improvement in your swimming, to get in swimming shape, and to make progress with swimming fitness and swimming technique or skill. With three workouts per week you are touching the water often enough to keep a good feel for the water, and you are working out enough to have substantial fitness benefits. At four workouts per week this continues, but the differences between two and three swims per week compared to three and four per week are smaller. The magnitude of the benefit gets a little smaller as you add more workouts.
Why does it seem to be better to swim more often, to do more workouts in a week, than to do longer but few workouts? Swimming is a highly skill-focused, repetitive motion sport. The more movements you can make the correct way, the better you get at swimming. When you swim for an hour, you may be getting tired toward the end of that workout and start to practice bad habits. If you swim more often but for shorter workouts, you are probably able to maintain better stroke technique for more of each workout. Voilà! Better swimming.