Getting a Snorkel

You need a snorkel to progress the quest and meet a special friend.
I compiled the information here for convenience.

Snorkel crafting plan

To get the Snorkel crafting plan, you need to raise your friendship level with Kuromi to Level 6.
This will unlock the story quest "Deep Diving", and upon completion you will get the Snorkel crafting plan.

Rubber

You can collect rubber on the island, but only a few will spawn per day.

Locations:

• You can look around Chococat's camp, near the bushes. Check the cave entrance nearby too.
  
• Look around the beach near Resort Docks.
  
• Check the area directly above the central Resort Plaza (but before you cross the bridge up north to the Cozy Islands).

Spark

The easiest way to get spark is to craft them.

You need to increase your friendship level with Pocaccho to Level 4, which will unlock the story quest "A Zipline Adventure". You will get the Spark crafting plan from Chococat during this quest.

You will need 3 Light Stones to craft a Spark. If you have befriended Kuromi from gifting presents, you should have plenty of them.

Ingot

From Exploration

The easiest way to find ingot is to explore the Gemstone Mountain area.
Some treasure chests will contain an ingot, like this one at the bottom left corner of the Gemstone Mountain area.

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From Crafting

Alternatively, you can craft an ingot with three iron. You can find the plan on a barrel at Gemstone Town.

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To obtain iron, you can find them at the Crystal Caves, head to Mount Hothead, or play the matching minigame at Gemstone Town.

• The Crystal Caves is accessible from Gemstone Town. It is a bit of a maze.
  
• Iron can be found at Mount Hothead around where you will meet Retsuko, or directly underneath it on the beach.
  
• The matching minigame is inside the yellow building at Gemstone Town, next to the building with the barrel. You need game tickets to play.

Crafting the Snorkel

Once you gathered all the materials, the final step is just to head to Chococat's tent area and use the crafting table.

[Conor]

rubber does respawn it takes a day

[Silvermoon]

Rubber will re-spawn but they're not always at the same exact locations :)

[Everybody do the flop]

Rubber doesn't seem to re-spawn, so you're going to have to get a ghost to grab some extra for you.