In order to monitor your workouts accurately, you are going to need a decent heart rate monitor. The Polar H10 Smart Heart Rate Sensor is an accurate wearable tool that helps you exercise in the right zone. It pairs with Polar Beat to give you access to real-time heart rate data on your phone. iOS and Android devices are supported.
The Polar H10 is compatible with GoPro HERO4/5 cameras, so you can overlay accurate heart data on your action cam videos. The Polar H10 is waterproof to 30m, so you can swim with it. It has built-in memory. Costs under $90 at this time.