A treasure chest can spawn at certain locations along the coastline or on the pirate ship wreck. There are 83 possible locations. At any point in time there are five treasure chests active.
When you find an unopened treasure chest you need to break it open to get the contents. Every chest contains gold coins which can be used to upgrade the builders desk to the final stage.
The treasure chest also has a small chance to contain one other item listed below (click on an item name for more information):