Thetis, a doctor, is always busy attending to patients, but has recently discovered a formula that allows merfolk to stay on land for extended periods without getting sick. She needs one more ingredient, which is some deep seawater that can be found in Tidewash Cave. This particular cave can be found to the northwest of Capstan, but you will need to cross the rift in order to get to it. The closest spot to cross is southwest of Hydropolis and once you do, follow the rift to the west and then north when you get to land. The cave is in a little inlet, which can be hard to see, since the sky will be dark.

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(1 of 2) The location of Tidewash Cave on the world map

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The location of Tidewash Cave on the world map (left), as well as the seawater you need to pick up inside of the cave (right)

There is a Trip Door at the top of the island with Tidewash Cave (Tidewash Cave), so activate that before you head inside. Head straight south, past the first group of enemies (there’s a chest in the same room with Determined Rakish Boots inside) and then west to find an area with a Dreamer’s Door. The item you’re looking for is in this section, as evidenced by the blue circle. You’re looking for a blue sparkle on the ground, which is on the southern side of the room, yielding a jar of Deep Seawater. Bring this back to Thetis in Hydropolis to finish the quest and gain a new citizen.