Before deleting the mod, use the in-game "magic" pie menu (shift+click your Sim with cheats on) to select Remove Fairy/Witch Lifestate to prevent your save from breaking.
Expect shark-like teeth and webbed hands. Expanded Mermaids
Place the mod files into your Documents > Electronic Arts > The Sims 4 > Mods folder. Ensure "Script Mods" are enabled in your game settings. Before deleting the mod, use the in-game "magic"
Mermaids can now use the bathroom or sleep directly in the water, making a full-time life at sea much more viable. Before deleting the mod