This page explains how to start and complete the Take Back the Sea side quest.
How to Start the Quest
Talk to Sebasto in Lurelin Village, which is east of Faron Tower, to start the quest.
How to Complete the Quest
Southwest of Lurelin Village, you can find a monster encampment in the bay between Aris Beach and Clarnet Coast. Kill all of the monsters on the encampment to unlock the treasure chest. Then go back and talk to Sebasto to get a silver rupee.