Here is another waterproof armband hard rate monitor that lets you closely monitor your performance. The RHYTHM 24 has a 24-hour battery life, five changeable training zones, and on-board data recording, including HRV. The device measures blood flow and movement to calculate heart rate and calories burned.
The RHYTHM 24 is compatible with RunKeeper, Strava, and other apps. It support Bluetooth and ANT+. More info is available here.