Sailing is risky and requires lots of knowledge
Just take a page from history. During the age of sail, specially during its first decades, explorers such as Columbus, Cabral, Magellan etc. had a hell of a bad time navigating the Atlantic. A major reason why they got so filthy rich was because sailing was a high-risk activity with large returns.
Sailing itself is not easy, specially with large vessels. For long distances, piloting knowledge is not enough - you need to plan for the currents and winds you'll face. Make a mistake, and you either bring too few supplies and die for need of them, or carry too many supplies which slow you down and become a logistics problem. Now imagine trying to plan for those without knowing where the currents and winds will take you, and without knowing how strong they are. Magellan and his crew nearly died of starvation when crossing the Pacific because of that.
Having good maps changes everything. Around the 1500's the Portuguese had a practical monopoly on marine trade with the Indies. They had been compiling maps of currents, winds, sandbanks and routes for centuries, but they kept those secret. Then by the end of the 16th century a Dutch dude called Jan Huyghen van Linschoten stole and published those. That had an impact in the 17th century orders of magnitude larger than Wikileaks had in our own time:
He is credited with publishing in Europe important classified information about Asian trade and navigation that was hidden by the Portuguese. In 1596 he published a book, Itinerario (later published as an English edition as Discours of Voyages into Ye East & West Indies) which graphically displayed for the first time in Europe detailed maps of voyages to the East Indies, particularly India. During his stay in Goa, Jan Huygen van Linschoten meticulously copied the top-secret charts page-by-page. Even more crucially, Jan Huygen van Linschoten provided nautical data like currents, deeps, islands and sandbanks, which was absolutely vital for safe navigation, along with coastal depictions to guide the way. The publication of the navigational routes enabled the passage to the East Indies to be opened to trading by the Dutch, French and the English. As a consequence, The Dutch East India Company and British East India Company would break the 16th-century monopoly enjoyed by the Portuguese on trade with the East Indies.
Seriously. You all would be fluent in Portuguese rather than English now if it wasn't for van Linschoten.
Your world can have its own Portugal, maintaining a monopoly on marine trade by maintaining a monopoly on knowledge.
Edit: maps are not the only secret they could keep. user3445853 made this comment:
The Dutch found the secret to preserving herring in a tasty fashion ("matjes/maatjes herring": remove all intestines except pancreas and pickle in a light brine --- pancreatic enzymes break down various structures in a tasty and stable way), and succeeded in keeping it secret for 300years (from just before 1400), selling the swedes and germans fish caught in their own waters and getting rich. This secret ANY uneducated deckhand could steal and sell, but they didn't succeed [easier steal than a detailed map set!]. So the maritime brotherhood CAN guard a secret.
This is specially interesting because many deaths on sea were caused by scurvy. Just so you know, when you've had scurvy for a while and you're still malnourished, every single scar you have in your body starts bleeding and your teeth start to fall.
So in your world, people can either pay the guild fairly and eat preserved food during their travels, or they can venture on their own not knowing where the wind blows, the directions of currents, and to add insult to injury they will only have biscuits, crackers and rats to eat.
