water running

Anybody have any tips for water running? I have an injury that is preventing me from running at the moment, currently supposed to be training for a 1/2 marathon that is in september. I have only water jogged twice, but I can't tell if i am getting the workout i'm supposed to be getting in the pool. I don't have a heart rate monitor, and my workouts usually consist of 2-3 minutes hard, and 1 min rest and repeat for 50 min. Anybody have any tips to make sure i'm getting the best workout for my training?

Thanks!