As you progress through the main story in Dave The Diver, you'll eventually stumble upon the Sea People Village where you'll first talk to their leader, Tenzhin. He'll ask you to do different quests for the people in the village so that you can gain their trust and start working on solving the main issue.
These quests will appear in batches, with the first one only having three. During the second batch, you'll get a quest in Mima's Restaurant to retrieve three Tube Corals so that she can improve her cooking. Here's how you can find these Corals for her.
How To Get Tube Corals
During the quest dialogue, Mima will mention that you'll most likely find the Tube Corals near the Whale Bones, which is a specific place that spawns in the Blue Hole Depths every time. To get there, you'll have to reach a depth of about 130 meters if you dive from the top. Alternatively, you can also teleport to the village and start there.
You can only go to the village or the Blue Hole Depths during the morning and afternoon dive.
The game doesn't let you go down there at night since you still have to open Bancho Sushi.
Once you go down about 130 to 140 meters, you'll see a tunnel leading further down, but it'll be blocked by Tubeworm. You can either get a UV light from a nearby yellow chest or equip the Sea People Necklace charm to get rid of them automatically. After passing the first tunnel from the top, you'll most likely see a giant structure of bones.
This structure can spawn in different places, so you might have to go through a few tunnels. When you reach the area, you'll easily find Tube Corals by looking for the blue marker on top of them. You'll only be able to find three of these, the exact amount that you need for the quest.
If you have another quest, you might mistake the Tube Corals for Lily Corals since they look similar.
Keep in mind that Tube Corals can only be found near the bones.