I have just bought the 5-6 player seafarers extension for catan and I also have the basic catan. Can I play like this? or do I have to have the normal seafarers?
Yes you need Seafarers to play the Seafarers 5-6 expansion as intended.
NOT a complete game! You need a CATAN® game (aka The Settlers of CATAN®), a CATANⓇ: 5-6 Player extension, and a CATANⓇ: Seafarers expansion in order to play with this game extension!