Firefly serves twice daily to Penang from Langkawi.

However one that has strong stomach can board to 2 1/2  - 3 hours boat ride from Jetty Point Kuah to Weld Quay Georgetown, Penang @ RM50 per trip. The ferry boards at 230 pm and 515 pm.

For the ones that have more times and look for sceneries, you can go to Kuala Kedah Ferry Terminal (1 - 2 hours) @ RM18 per trip boards hourly from 730 am then take taxi or bus to Alor Star (Komp. Bas Shahab Perdana) to go to Butterworth. Bus leaves every 15 minutes and takes approximately 2 hours. From Butterworth, there is a nice short ferry ride to Georgetown @ RM1.20. 

Another route is a boat ride to Kuala Perlis Ferry Terminal (45 minutes - 1 hour) @ RM15 per trip boards hourly from 730 am and then take taxi to Kangar Bus Station to board the bus to Butterworth (via Alor Star).