The Hero9 "GoPro labs" firmware also lets you burn-in the current speed into the video. It displays in M/S but could be useful for skiers that run multiple speeds and might not be sure what speed a given pass was run at.
Also, the Hero9 has the removable lens for the "Max lens mod" which makes the viewing angle wider- so it is not useful at all for waterskiing, but now that the camera has a removable lens I assume that before long someone will make an aftermarket mod lens that does the opposite to give you an optical zoom that snaps on easily.