To upgrade Argosy Submarine Slots in Monster Hunter Rise, you must first unlock the Cultural Exchange application, which allows you to find Wisplanterns and Boatshells. This page explains how to find those materials in Monster Hunter Rise.
How to Unlock the Underwater Cultural Exchange Request
Complete the 3-Star Village Urgent Quest (unlocked after completing the required 3-Star Key Quests) to get the Cultural Exchange Request. Talk to Rondine the Trader at The Argosy at Buddy Plaza after running this Urgent Quest to get this Request. You may also be able to unlock this by progressing through the Gathering Center, although we’re not sure how at this point.
You must have this request before collecting items for it to count.
Where to find Wisplanters
Wisplanterns can only be collected in the Sanctuary Ruins, and they are considered account items that are automatically delivered for Kamura points, so you don’t have any in your inventory yet. They can be found by searching for Shimmering Red Berries in Shrine Runes, which grants both the rarer Firelanterns and Wisplanterns.
Where to find ship shells
Boat shells can only be collected in the Frost Islands, and like Wisplanterns, they are elements of the account. Find them by searching Oyster beds. Ship shells are the rare material in this hangout.