Well, the way you should look at it should be based on your heart rate and PLE (Perceived Level of Exertion) and duration of activity as opposed to pure ‘distance’.

For example, if you run at an aerobic pace with 10 minute miles for 6 miles, your caloric burn would be around 700-800 calories — that is assuming you stay in Zone 2 or 70-80% of your max heart rate. If you swim, your Zone 2 would probably be around 10 BPM slower for the same level of exertion (your body does not have to cool down or fight gravity) however you are using more muscle groups so the caloric burn is about the same.

In summary, 1 hour of Zone 2 running is the same as 1 hour of Zone 2 swimming. As swimming is based 70% efficiency and 30% fitness, the distance you cover has much more to do with technique than fitness. IF you can swim 3000M in one hour and feel just as fresh as running 6 miles easily in that same hour then for you, 6 miles of running = 1.8 miles of swimming. If on the other hand you can run 8 miles easily in 1 hour but can barely swim 1000M kicking and sputtering in that amount of time then 8 miles running is the same as .6 miles swimming.

Read the Terry Laughlin book and you will find there’s a triathlete hiding behind that aerobic jet engine you use for running.

See you at the starting line.
References :
Total Immersion by Terry Laughlin
Going Long by Joe Friel
Total Heart Rate Training by Joe Friel

Training for 1/2 and full distance triathlons.