How to see temperature of water when you doing freedive?

I do not have the G1, so am not sure whether you can customize the data screens of the apnea activity profile. If you can, then it is easy - just add the temperature field to one of the screens, or add a new screen.

If you cannot customize it, then escape from the activity by pressing the SET button (lower right) or (if that button has already an assigned functionality, in the apnea activity) assign a hotkey to go back to the watchface, and use it. Once on the watchface, scroll down to the temperature widget, and there it is. You do not have to do it underwater, just prepare the temperature widget when you start the activity, and as along as it runs, the widget stays in the background, and you can come back to it by a single button press.

When in Apnea mode, push Down button to change screen to temaperature.