Ocean Doors are one of those resources in Pokémon Pokopia that will stop your main story progress cold if you don't know where to look. The Bubbly Basin DLC, part of the first wave of the Expansion Pass, requires them to complete a specific request, and the game doesn't exactly hand you the recipe on a 
plate.
The short version: you won't find Ocean Doors sitting in the overworld. They have to be crafted, and before you can craft them, you need to track down a hidden recipe. Here's exactly how to do both.
Where do you find the Ocean Door recipe in Bubbly Basin?
The recipe doesn't have a single fixed spawn point, so don't stress if your location looks slightly different from what others describe online. What matters is the general area: head above the entrance to the cave where the Pokémon Center is located in Bubbly Basin.
That's the same spot where 
Popplio's request sends you to construct a large building. To make room for that construction, you'll need to clear out a range of blocks in the surrounding area. The Ocean Door crafting recipe is tucked away inside one of those blocks, hidden inside a glowing spot. If you don't find it there, keep an eye out for a whirlpool drop in the same general zone, as the recipe can also appear that way.
Clearing the area for Popplio's 
build request and hunting the recipe at the same time is the most efficient approach since you're doing both jobs simultaneously.
Popplio's construction and didn't notice a glowing spot, check for whirlpool drops nearby. The recipe can appear through either method in the same area.How do you craft Ocean Doors in Pokémon Pokopia?
Once the recipe is in your hands, crafting Ocean Doors is straightforward. Each Ocean Door costs exactly 1 Star-Spotted Rock, making it one of the cheaper crafting recipes in Bubbly Basin.
Star-Spotted Rock is found in abundance throughout the Bubbly Basin area, so you're unlikely to be short on materials. If you've already been working on Popplio's construction request and clearing those blocks, there's a good chance you've already collected more than enough to craft the 2 Ocean Doors the story requires.
Popplio's request, you'll almost certainly have enough to craft both Ocean Doors without any extra farming.
Star-Spotted Rock in inventoryWhat are Ocean Doors used for?
Beyond completing the specific story request that requires them, Ocean Doors are a building item you can place freely in your construction projects across Bubbly Basin. Once you've finished the required request, any extras go straight into your building toolkit for customizing the underwater area however you like.
Getting Bubbly Basin fully operational opens up a lot of the DLC's content, from new Pokémon habitats to fresh crafting materials. If you're still working through the early steps of the expansion, the guide to unlocking Bubbly Basin covers the prerequisites you need before any of this becomes accessible.
Making the most of your time in Bubbly Basin
Ocean Doors are just one piece of the Bubbly Basin puzzle. The DLC packs in a lot of new content, and tracking down every item and recipe can take some digging. For a full breakdown of every Pokémon added to the expansion and their Specialties, the Bubbly Basin new Pokémon and Specialties guide is worth bookmarking before you get too deep into island planning.
The Starmie Doll is another hidden item in the same expansion that trips up a lot of players, and it unlocks Phione along with access to resources from Dream Islands. That's a separate hunt, but it follows a similar logic to the Ocean Door recipe: the item is hidden, not sitting in plain sight.
For everything else the expansion has to offer, the full Pokémon Pokopia guide collection has you covered from habitat requirements to crafting unlocks.