Catan: Seafarers (previously, The Seafarers of Catan) was released in 1997 as the first expansion to The Settlers of Catan. The expansion incorporates ships and exploration into the game, elements that were deemed too problematic to implement when Klaus Teuber first released Settlers, but were originally planned. The expansion is currently on its 4th English edition as produced by Mayfair Games. It can be extended using the Catan: Seafarers 5-6 Player Extension. There are also a number of independent scenarios designed specifically for Seafarers.
From the 3rd Edition packaging:
Ahoy! You need The Settlers of Catan boardgame in order to use this expansion.
In The Seafarers of Catan you control a group of seafaring settlers exploring and taming the uncharted Isles of Catan.
Embark on a quest to settle the island, build ships, and chart the nearby waters. Guide your settlers 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 combinations of resources - grain, wool, ore, brick, and timber - to develop your ever-expanding empire.
Of course, the best strategy and a dash of luck decides who will be the undisputed ruler of the Isles of Catan.