Hello there,
After further investigation into the reports that were shared with us following the release of TU 1.5.0, it has been discovered that these reports were different issues than the one being investigated in this thread ("Deviled Water" not registering as complete after the NPC enters the water). We will now be marking this thread as resolved again, as we have had no new reports of this issue persisting in-game.
If you are encountering a different issue in "Deviled Water" that is preventing you from completing this World Event, then I recommend you check our out additional Megathread for this World Event. There is a detailed update within that thread about the currently ongoing investigations with this side quest, as well as the status of the ongoing investigations.
Thanks!