You can manually trigger the lake scene with a scripting mod (u/Tiki_Man_Roaron on Reddit figured this out):

• Install Script Extender with Console (manually or from Nexusmods)
• Start game, it should launch a text console in another window
• Load savegame, travel to Rivington
• Focus console, press enter, insert the following command and confirm with enter again:
  • Osi.PROC_CAMPDEBUG_QueueNight(“NIGHT_Shadowheart_Skinnydipping_9f583304-0a1a-498c-acf9-3c8dcc30ee3d”, 1)
• Start long rest in Rivington

You should see the exclamation mark above her head, and the scene should play after talking to her and going to sleep. If you want you can remove the Script Extender mod afterwards.