I swam 3-4 times a week last summer. It rocked, better then any other cardio. You get a full body workout in 30 minutes and don't have to do anything else. I havent been in the pool much this winter becuase the pool at our 24 fitness is never consistant in its temps so I figured I would wait till spring summer to start swimming again.
You can just swim laps, I usually will swim 16 laps, which is a mile in our pool, may be different over there.
You can also grab one of the little boogie boards, hold it out in front of you , and kick all the way down. Keep your legs tight and use your whole leg, dont bend your knee's. After a few laps it will burn.
I sometimes will just go down to one end with my arms, no kicking, and return with just legs kicking and now arms. That way you use your whole body, and it makes it a little harder.