To unlock fishing area it is important that you complete all previous fishing quests first. You can receive these quests from fisherman Oleg, who can be found on all three of your farms.
Please note that some of those quests might be not fully related to fishing (e.g. collect wheat).
The last quest that you need to complete is titled "Patience is silver, but hooks are golden!" and asks you to buy and use a large Minnow hook. After completing this quest, fishing area and dockyard will be unlocked.