Diabetics have a sensor on their bodies that speaks Bluetooth. It talks to their phones (running eg. xDrip). The phone then communicates this to the watch.
My wife is a diabetic and this kind of stuff, while it seems minimal, makes a big difference for her quality of life.
It's all about building these things by yourself. The tools are there, and Pebble used to be the first smartwatch every open source diabetic I know used (including myself).
If you're not familiar, here's a good starting point:
Oh, and you want to avoid Apple's hardware. Some of the software can only be distributed as source code, so installing them to an iOS device is not easy.
What am I missing?
(Vested interest as a diabetic myself.)