Temtem is filled with small quests that you can complete as you play through the game. The first one you will find is if you go to the beach in Zadar, the village where you start the campaign. Here you will find Clara, who is looking for her pendant. She lost it while swimming, and you can offer to help her.
The clue to where to look comes up in conversation, as you can ask about the currents, and Clara says it may have been carried down the river. You will need to head for Brica de Mar, but be advised that completing this quest is locked for a while. Just continue through the story until you reach Sophia’s Dojo in Arissola and are told she is not there. Then stay exploring the world until you arrive at Windward Fort, where you will find Sophia trapped in a cage. When you see her, continue towards the fort, then head to the top where you will battle Lady Lottie. Beating her will get you the Windward Key, which will then open Sophia’s cell.
Now, you can return to Arissola and fight Sophia at her Dojo. Beating her will finally get you the Surfboard. Head back to Brica de Mar and use the Surfboard to cross the river at the docks. On the far side, search the cost until you find a glittering area, and this is where you will find the pendant. You can now return it to Clara at Zadar.
For finding her pendant, Clara will give you an item called TC001: Tsunami, which can be used to teach the move Tsunami to your Temtem.