I recently moved to a SS SB Apple watch from a Sport and notice horrible battery life for until last night when I wen through the Apple Watch app and notice that some of the apps were still "installing" even though they had already been installed. So I uninstalled and reinstalled.
It seems to be a bug in unpairing and pairing process. that some apps stay in the "installing" phase indefinitely draining the battery. I could be even worse if you are testing apps with Apple Watch functionality via TestFlight as take longer to install and have to be managed in the TestFlight app.