Starting from Apple Watch 6 and watchOS 8, Waterspeed can always stay on and show data even if the wrist is not turned.

Usually, this feature is automatically activated. In case it doesn't work, please check in System Settings, Display, Apps, Waterspeed.

Another issue can be caused by "Wrist detection" setting: in this case, if the Apple Watch loose its contact with the skin, it will shut off the screen after a few seconds.