HIIT workouts let you burn more fat and keep your body on its toes. You can always use an app to try these routines. Moov HR is a smart wearable device that serves as a coach for high intensity interval training. It is waterproof and lasts up to 6 hours on battery.
The device can be attached to your sweatband or swim cap to give you feedback on your performance. It pushes you to stay in the right fat burning heart rate zone. How does it work? It measures your heart rate from your head using LED light. More info is available here.