Catan and Seafarers 5-6 player expansion
You need both player extensions in order to play with 5-6 players on Seafarers.  The Seafarers 5-6 player extension doesn't contain the pieces needed for the extra players.  You'll need the frame pieces, extra titles, houses, roads and cities from the regular Catan 5-6 player extension in order to play Seafarers with 5-6 people.