According to Doug Stern, who teaches pool-running classes for the New York Road Runners Club, his students not only maintain fitness while water training, they sometimes improve it as well.
Train with a purpose: "You can't get in the water and space out," says Stern, a nationally ranked swimmer and triathlete who has run 18 marathons. He recommends using a flotation device to suspend yourself in the water, even if you're a good swimmer. This allows you to work out longer thus, more productively.
Try intervals: Almost any type of training you do on the roads can be duplicated in the pool, Stern says, but interval training provides the most bang for the buck. Best of all, you can get a great pool-running workout in 30 minutes or less.
Follow the pool workout: First, warm up with at least five minutes of easy "running." Then comes the tough stuff. Stern usually has his runners go hard/ easy in a ratio of three to one. Example: 45 seconds of hard effort followed by 15 seconds of recovery, repeated 10 times.
To increase difficulty during the hard segments, you can either increase your cadence or your stride length. If you're pretty tired by the end of the session, you may want to do your five-minute cooldown by "power walking" in the shallow end. Finish with some easy swimming, followed by light stretching.