I've been swimming 3 times per week for the last month and a half. I recently underwent knee arthroscopy on both legs and it is a part of the rehab.
I used to do competitive swimming when I was a kid, training 2 hours, 5 times a week, averaging maybe around 4.5 km if not more when I was only 11yo.
Now swimming is 1000 times harder for me. I still have the right technic but my body has changed and it just doesn't float anymore! My legs feel like they are made of lead and it feels like I use 50% of the energy just to stay on the surface and maintain a good horizontal position and only 50% to move forward. It is funny but when I take the kick-board and just work the legs even the 45yo women using the kick-board swim faster! After 100m it feels like my legs are going to break and everybody else seems able to do it for days! I swim 2-2.5km and it just kills me! And my HR is sky high the whole time so this feels like good exercise.
Also, I've gained around 8 pounds of lean muscle mass since I started swimming and it actually made swimming harder for me. I just feel like I don't get enough air and get winded faster. My guess is it has to do with needing more oxygen for the extra muscle.
So yeah, it feels like general-fitness-wise my cardio is getting a good workout and I am sure at least some of this transfers to any sport. I might go as far as considering substituting 1hr running for 1hr swimming two or three times a weeks once I'm all healed, since it has the added benefit of being low impact on the joints. But for sure as soon as I can, I'm going sprinting