Reference article
Cove Treasure Hunt
Follow clues and dig up buried chests to find the Quackbeard’s Cove Tournament Ticket.
Guide
Talk to the Mallard Gang and win the races needed to obtain the shovel and first clue. Read the clue in your bag, find its dig site and open the chest. Follow the next clue to the buried map, then use the map to find the final chest.
Reward
The final chest contains the ticket for 
Quackbeard's Cove Tournament. Keep opening each unearthed chest before moving on to the next clue.