Will Paddle Logger suck the life out of my iPhone's battery?

No! 

While we cannot guarantee that using the app will not drain your battery faster than if you weren't using your device at all, we have found in testing that actually provided you remember to lock the screen after you start the app, it doesn't really have an effect! When operated correctly we have managed 7-8 hour trips comfortably.

It is also worth pointing out that if you use your Apple Watch and are concerned about battery life, when your Watch is in the vicinity of your iPhone it intelligently uses the GNSS chip in the iPhone and conserves battery life. So we recommend taking your iPhone stowed in a waterproof case, this also has the added benefit of improved location data and having a means of communication on you should you get into difficulty.