From the 3rd edition packaging:
In The Seafarers of Catan 5-6 Player Extension you control a group of seafaring settlers exploring and taming the uncharted Isles of Catan. Embark on a quest to settle the islands, build ships, and chart the nearby waters. Guide your seafarers to victory with clever trading, cunning development, and by establishing the most productive sea lanes. Acquire your resources through card play and the luck of the dice. Discover far-off mines and use gold and a combination of resources to develop your ever-expanding empire. As always, the best strategy and a dash of luck decides who will be the undisputed ruler of Catan!
Now five to six players can sail into the uncharted and explore and settle the mysterious islands near Catan! This 5-6 player expansion for Catan: Seafarers allows you to add 1-2 more opponents without sacrificing ease of play. Try one of ten new exciting scenarios! Designed for 5-6 players, it adds even more drama to the award-winning game of seafaring, exploration, and trade.