To upgrade Argosy Submarine Slots in Monster Hunter Rise, you need to first unlock the request Cultural Exchange, which tasks you to find Wisplanterns and Boatshells. This page covers how to find those materials in Monster Hunter Rise.
How to Unlock Cultural Exchange Submarine Request
Complete the 3-Star Village Urgent Quest (unlocked after completing the required 3-Star Key Quests) to get the Request Cultural Exchange. Talk to Rondine the Trader at The Argosy in the Buddy Plaza after doing this Urgent Quest to get this Request. You may also be able to unlock this by progressing in the Gathering Hub, though we’re unsure of how at this moment.
You must have this request before gathering the items for it to count.
Where to Find Wisplanters
Wisplanterns can only be gathered in the Shrine Ruins, and are considered Account Items that are automatically turned in for Kamura Points, which is why you don’t have any in your inventory already. They can be found by searching Shimmering Red Berries in the Shrine Runes, which grants both Firelanterns and the rarer Wisplanterns.