Is it possible to develop third party apps using Apple's WatchKit with an API for heart beat/rate data access/raw data access?
Is the WatchKit SDK publicly available for developers now?
Heart Rate Raw Data information is now available in Watchkit for watchOS 2.0.
WatchOS 2 includes many enhancements to other existing frameworks such as HealthKit, enabling access to the health sensors that access heart rate and health information in real-time.
You could check this information in the following session which is total 30 minutes presentation.If you do not want to watch entire session, then you directly jump to Healthkit API features which is in between 25-28 min:
WatchKit for watchOS 2.0 Session in WWDC 2015
Here is the source code implementation link