There are a total of 15 treasure chests in this game, 5 per Island. The first one you will come up against will be behind a wall of sand you cannot current get past.
Once you have rescued Wayne you will gain the ability for a new minion type who can dig thorough this wall of sand. Go back to this wall and send your wolf minions to dig through it giving you access to the first treasure.
The chest is within a Clam so you can obtain another achievement hear called Pearl-fect Timing. Send your minions into the clam after it has re-opened to ensure it does not shut on them and take this chest back to an Impa portal to collect the chest and unlock this achievement.