Oxygen Bottle hasn't shown up in crafting menu

A friend of mine has started playing raft, and is now up to caravan island. At this point, an oxygen bottle would be really helpful for the dive.

I'm much further in the game than him, and don't remember when the oxygen bottle shows up for crafting. I've looked online and not found anything. What events need to happen in game for the oxygen bottle to show up in the crafting menu, or would it appear my friends game is bugged in some way?

In case it matters, the flippers are in the menu.