For those of you wondering when this is suppose to unlock and convinced that you have already completed all beach quests (blue flag), hopefully this will help clarify.
There are more quests added to each area as you progress in the "story". You need to continue with the story by doing all available missions (exclamation point) which will unlock new areas and more challenging quests in the previous areas. Below is a list of quests which are only all available after you unlock:
Below are the 10 Beach Quests you need to be victorious in order to unlock the Sailor achievement.
1. Welcome to Rune Island (1000)
2. Sunny day at the beach (1000)
3. The abandoned lighthouse (1000)
4. Inside a summer storm (1000)
5. Fight for the cliff (1000)
6. A journey to the secret bay (1200)
7. Mysterious shipwrecks (1200)
8. Deep into the whales' cemetery (1200)
9. Smugglers cave (1500)
10. An empty chest (2000)
Hope this was helpful! InjuryProne19