HRM underwater

FluffyMamaNeeds2Lose

I got a Polar Ft4 HRM and it works great in the gym however even though it says that its water resistant it doesn't register my heart rate while I'm swimming or doing water aerobics. The watch kept saying check the heart rate transmitter. Anyone have the same issue? Is there a HRM that actually works under water? I do a lot of workouts in the water and would like to see the calories I've burned.