Hello Laura,
I think swimming is considered a cardio workout more than strength, but it will build your upper body if you do it hard enough. But generally, it increases the heart rate and therefore cardio.
I have combined weight training with swimming in the past. I would do one day swim and one day weight training. I have also read of some people combining swimming and weight training on the same day, but it didn't say what body they worked on. I couldn't do this because I didn't have enough time in the day to go from the pool to weight training at home.
Maybe you should give it a try and see how your body copes? I would probably not do a heavy upper body workout following a swim though, legs is probably better.
There's not much that is right or wrong in training terms, only what does or doesn't work for you.
Yen