If you need the heart of the sea to make the conduit, you will need to find buried treasure, so here's where and how to find it in Minecraft.
Where to Find Buried Treasure in Minecraft
To find buried treasure, you will first need to locate a treasure map in Minecraft. You can find these maps located in chests in shipwrecks; you can find these shipwrecks underwater in ocean biomes. However, they can sometimes spawn on the beaches and rivers as wrecks, but they will be underwater in oceans more often than not.
Once you locate a shipwreck, you will need to search the wreck for a treasure chest containing the treasure map, and each shipwreck can also have up to three chests, so you might need to check them all to find the map.
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That said, once you located a map from a treasure chest, put it on your hotbar. When it is on your hotbar, press the slot it is in, and you should see something like the image below.
This is how the map looks in-game, and if you follow the map to the red X, you will find buried treasure. You can also figure out the direction you need to go in by the white arrow on the map, as this arrow indicates the direction you are facing.
So if you want to find the buried treasure move around until the arrow is facing where the red X is on the map and head in that direction. Once you reach the area where the red X is on the map, you will need to dig down. Soon or later you will find a chest, and inside it will contain some various loot items, including the heart of the sea.
For more on Minecraft, we at PGG recommend checking out both What is the Range of a Beacon in Minecraft? and What Does the Hero of the Village Effect Do in Minecraft.