Rowing is actually great for cardio. Once i'v finished my power program/leg leg prioritization, i'll cut the most heavy weights off and start rowing 3-4 times a week. I'v usually done my cardio by playing badminton/tennis/football + running, but that rowing is definitely something i'll look into, especially when we got one of those machines where it work at. There's some kind of mini goal to get that 500m test under 1.30, had 1.34.6 after couple times of rowing and learning the technique but as summer comes and football/tennis becomes more of my thing, i don't need to be doing gym 5 times a week with the heavy(or heavy for me) weights, rather do more functional training and add different elements to it, which i think rowing is quite clear to be in it. If im not mistaken, it's around 80% of your muscles that are working when rowing and if you really want to get exhausted, even doing that 500m at fully speed will kill you. Probably some sort of longer distance once a week, 10x30-60sec intervals couple times a week and then some pyramid with growing resistance, something like 30sec with each resistance from 1 to 10 and then from 10 to 1.
If somebody fucked up his knee, rowing should more than fine for you.