Aground

Building the dock is part of the main series of quests. During early play you will be told of goods being stolen from storage when you open it up. Eventually you will catch the thief red handed and she gives you a proposition - help her open a trading post on your island. The post can only be built in a specific location to the west of the island, so you'll need to demolish anything you might have built there.

Once done, the thief/merchant will task you with the construction of the dock. The dock will allow you to build a boat to sail to the western island. You will need money to build the dock, which can be obtained by selling goods to the traders you have previously built.