Clearing in & out of Langkawi is easy, no hassle, and it is free of charge.

The Officials are helpful and will not ask for your money.

If you have been in Thailand before and had a bad experience there - relax.
Also, unlike Thailand, there is NO LIMIT for the length of stay for your boat.
(See full comments on this below).

If you approach Langkawi from the east and arrive near Kuah, go to Jetty Point, 
(where ferries depart for Penang, Kuala Kedah, Kuala Perlis and Satun)  

If you approach from the west, you can clear in at Telaga Harbour Park.
(The headquarters of Immigration is also situated there).

It makes no difference which location you use to check in, or check out.
Both are efficient and free of charge. (See open/close days & times below)