I'm practicing a martial art and I'd like to work on my cardio. To do that most people run, but I've had knee problems so I'd rather pick up swimming instead (I also enjoy it more to be honest...). I'd say I already have a decent swimming technique.

My question is this : how should I train ? Should I go for interval training with short, fast distances in crawl with recuperation periods, or just swim long distances in crawl ?

Bearing in mind that my main goal isn't to progress in swimming itself, but rather to work on my cardio (primarily) and maybe along the way buff a little. 

I think I'll be doing besides swimming two, maybe three practicing sessions of my martial art in a week, and if I have the time go to the gym once a week.

What do you guys recommend ?