Copper can be harvested deep underwater around Islands with a Hook. It can be found along the cliff walls below the Island. It looks similar to Metal Ore, but has a distinguishable red tint. Flippers and an Oxygen Bottle can help the process of gathering Copper. The ore can be put into a Smelter and will turn into a Copper Ingot after 80 seconds.
A specific Large Island has a treasure trove of Copper and Metal Ore.
See the video below.
- Can be used to create Copper Ingots using the Smelter.